Priest powerlevelling guide
Some background info about me:

I play on a PVE server on the Alliance, so this guide is slightly biased toward those as such. But I think it offers some good grinding spots regardless, and a lot of the later zones in the game are neutral anyway (which may be a bad thing, if you're on a PVP server).

I've been playing WoW for only two months, but I levelled my first character (a rogue) to L60 in about 11 days /played, and this Priest once I finish it will have just under 8 days /played.

My style of play is impatience. I'm impatient to level, I want to be 60 and raid, and I don't like wasting time doing laborious quests. I think grinding is the fastest and the most profiting way to L60. Shadowpriesting is the only way to powerlevel a priest.

I do not like to run instances pre-60 because the gear doesn't really matter in the long run, especially if you are powerlevelling. You'll be levelling so fast that you'll make your newfound blues obsolete in a hurry. The only instance I ran on my Priest was Deadmines. BFD wouldn't be a bad one, though. If you want to instance, then don't let me stop you. Even better, see if you can get a guildy or two to run you through.

All of my characters have been Alliance, so this guide is probably slightly biased as such.

My advice on talents:

L10-14: Spirit Tap (5/5)
L15-16: Imp SW:P (2/2)
L17-19: Blackout (3/5)
L20: Mindflay (1/1)
L21-22: Blackout (5/5)
L23-24: Shadow Focus (2/5)
L25-27: Shadow Reach (3/3)
L28-29: Shadow Focus (4/5)
L30-34: Shadow Weaving (5/5)
L35-38: Darkness (4/5)
L39: Vampiric Embrace (1/1)
L40: Shadowform (1/1)
L41: Darkness (5/5)
L42-43: Imp Psychic Scream (2/2)
L44: Silence (1/1)
L45-49: Unbreakable Will (5/5)
L50-54: Wand Spec (5/5)
L54-59: Mental Agility (5/5)

Your three most important talents as a Shadowpriest are (1.) Spirit Tap (2.) Shadowform (3.) Vampiric Embrace. Spirit Tap is what makes grinding as a Priest so fast and easy. Shadowform by itself is a bit lacking (since you can't cast Holy spells to heal yourself) but Vampiric Embrace circumvents this difficulty.

You can learn Vampiric Embrace whenever, but I think it's best to get it pre-40 than at L41. Honestly, Silence is an optional spell and I never used it on my Priest until about L48 or L49, but it is HIGHLY useful in certain circumstances so I would recommend it. Wand Spec is a pretty nice talent for some added damage.

My usual attack pattern:

Before Shadowform:
Mind Blast > (start backing up) > SW:P > Regen (optional) > Mindflay > Mindflay > Mind Blast > Wand

After Shadowform:
Mind Blast > (start backing up) > SW:P > Vampiric Embrace (optional) > Mindflay > Mindflay > Mind Blast > Wand

If Blackout procs early in the sequence, you can sneak in a third Mindflay if you want. This pattern might not work on some faster enemies (many beasts run faster than many humanoids, so you probably only have time for one Mindflay).

It is important to wand your enemy to death so that you'll end the fight having already regenerated some mana. Your wand is probably your single-most important piece of equipment. I found this especially true at lower levels.

While I am grinding, I never use PW:S (unless I'm in large danger) and I never use PW:Fort. Fortitude costs too much mana and won't help your grinding any. If you are grinding well (read, efficiently and fast) then you will probably never be at full health anyway. And typically, mobs that you grind are weak, easy kills. So I never used PW:F, and instead of casting PW:S I just used regen/VE to heal myself.

My advice on gear/stats:

Your most important stat by far is Spirit. I recommend sacrificing other stats (e.g., INT/STM) for as much SPR as possible. The faster you can regenerate your health and particularly your mana, then the less downtime you have between fights and the faster you level. With Spirit Tap, you'll get even more mileage out of SPR.

As far as INT and STM, I would rank INT more important than STM. If you really want to, you can boost your STM with Fort. As a Priest, your mana pool is your lifeline. If you have mana, you can shield yourself to prevent more damage, you can regen/heal yourself to recover life, and you can deal large amounts of damage.

Honestly, neither one was important to me. I grinded too fast to ever reach full health/mana.

Some gear to look out for as you level:

Deadmines: http://thottbot.com/?i=2273 (Staff Of Westfall)
Deadmines: http://thottbot.com/?i=5989 (Corsair's Overshirt)
Deadmines: http://thottbot.com/?i=6502 (Cookie's Stirring Rod)
Blackfathom Deeps: http://www.thottbot.com/?i=2321 (Gravestone Scepter)
Gnomer: http://www.thottbot.com/?i=2293 (Civinad Robes)
Stranglethorn Vale: http://www.thottbot.com/?i=38437 (Enchanter's Cowl, pattern sold in Tailor Shop in BB)
Scarlet Monastery: http://www.thottbot.com/?i=3228 (Illusionary Rod, overrated IMO)
AH: http://www.thottbot.com/?i=3368 (Dalewind Trousers, usually run about 20g on my server)
AH: http://www.thottbot.com/?i=6760 (Soulkeeper, usually 30-40g on my server)

My advice on professions:

Honestly, it really doesn't matter. Do whatever is needed by your guild, or do whatever you want to do and whatever you think is fun.

My personal choice is Tailoring/Enchanting. You can powerlevel Enchanting with the greens you make. A lot of your grinding will be against humanoids, so you'll have lots of cloth. With enchanting, you can make wands for your early levels. Late game, it has the potential to be very profitable.

Also consider alchemy and engineering, though. Alchemists can create potions (which may not be terrible useful to you as a healer), but the transmute ability is very useful. Engineers are suited for PVP, but they also can make cloth head-pieces.

My advice on where/how to level:

The part you've probably been waiting for. For your first 20 or so levels, just do quests. Since I was alliance, I starting doing Deadmines at about L19, and I did DM several times for decent EXP.

Summary: Do zero (well, almost zero) quests from ~L30 to ~L52. Then quest like hell until you ding at 60.

When: L21, L22, L23, L24
Where: S.E. Duskwood (Tranquil Gardens Cemetary)
Mobs: L21-23 Skeleton Warriors/Mages
Comments: The area is dense with mobs and has a fast respawn rate. Should get ~18,000 EXP/hour here. There is a grind quest for this area, "The Night Watch." Pretty close to the graveyard.

When: L24, L25, L26
Where: N.W. Duskwood (Raven Hill Cemetary)
Mobs: L23-L25 Skeletal Fiends/Horrors
Comments: If this area gets crowded, you can always jump over to the Ghoul area north of the Skeleton area. Watch out for Mor'Ladim. The second "Night Watch" quest is here. Raven Hill is rather far away from the graveyard; take special care not to die.

When: L26, L27, L28, L29
Where: Duskwood
Mobs: L27-L29 Ghouls (Raven Hill Cemetary/Catacombs), L26-30 Ogres (Ogre Mound)
Comments: I'm sorta at a loss about what to say for these levels. Personally, I did a lot of Duskwood quests here. The Worgen quest gives a 24.2 dps wand (if you skipped BFD) and the Bride of the Embalmer quest gives a 7 INT/7 SPR pair of shoulders. Do whatever for these levels, but if you're going to quest it up in Duskwood, do bunches of quests at a time to minimize traveltime. Set your hearth to Darkshire, also.

When: L29, L30, L31, L32
Where: N.E. Duskwood (Manor Mismantle)
Mobs: L29-30 Fetid Corpses
Comments: There are two treasure chests here (one in the middle of the area, one in the house). Be careful of Stalvan inside the house; try to avoid him since he curses you. You'll need to remove the disease these zombies give you. You'll get lots of wool and a bit of silk from here. Pretty close to the graveyard.

When: L33, L34
Where: E. Desolace (Kolkar Village)
Mobs: L31-34 Centaurs
Comments: There is one grind quest here for Centaur Ears, and the quest reward is a 7 SPR ring. These guys drop tons of silk and a bit of mageweave. For me, L33-36 were the slowest and most painful. I'm wondering if there are better places to grind than in Desolace (e.g., Arathi trolls perhaps?). If you want my personal opinion, avoid STV like the plague. It was a slow and painful zone for me. Not far from the graveyard in the center of the zone.

When: L35, L36
Where: S.E. Desolace (Magram Village)
Mobs: L32-L36 Centaurs
Comments: This can be a tough area to grind in. I was able to get into a groove, but you have to find the right spot. For me, I used the spot on the map where the land juts into the lake to form a peninsula. There is a small tower near here, and sometimes I crossed the bridge and killed the mobs on the island. Beware and avoid the Pack Leaders at all costs. They are very frustrating, and you'll have to figure out their movement paths so that you can avoid them. Not far from the graveyard in the center of the zone.

When: L37, L38, L39, L40
Where: N.W. Badlands (Dustbowl)
Mobs: L37-L39 Lesser Rock Elementals
Comments: BADLANDS IS AMAZING! Your greatest grinding will probably take place here. Your mount money will come from here. Elemental Earths sell for 75s-1g *apiece* on the AH on my server, and stacks of 20 Solid Stone sell for 1.5-2g. Make use of the auction house! At L40, I had about 200g thanks to Badlands. The Elemental Earths are also good if you are a tailor (powerlevel your tailoring skill with the Earthen Vest pattern. There is also a grind quest for the Lesser Rock Elementals. IMPORTANT NOTE: Make sure you are NOT using a nature-based wand. Elementals are IMMUNE to nature damage. If you are an enchanter, you can craft yourself a Greater Mystic Rod (L175). Right near a graveyard if you die.

When: L39, L40, L41, L42
Where: S. Badlands (Mirage Flats)
Mobs: L39-L40 Rock Elementals
Comments: These guys are located very south in the mountainous area. Another grind quest here. You'll continue to get fast experience and make big money here. The only downside to grinding these Elementals in Badlands is that you'll be gimped on Mageweave cloth (if you're a tailor). At L40, your EXP/hour will shoot up to ~30,000.

When: L42, L43, L44, L45
Where: S.W. Badlands (Camp Something)
Mobs: L42-44 Greater Rock Elementals
Comments: There are two areas with these guys, one to the east of the Ogre-cave and one to the west. The eastern area in the hills
is the larger of the two and works better for grinding. The area is circular, so you can just do laps. If the eastern area becomes crowded, you can switch back and forth between the two areas easily enough. Also, there is yet another grinding quest here. Right near a graveyard if you die.

When: L45, L46, L47
Where: E. Tanaris (Lost Rigger Cove)
Mobs: L44-L45 Southsea Pirates
Comments: After nine levels in Badlands, you finally get some change of scenery. This area can be quite crowded, so you'll need to be fast. As a Shadowpriest, you should kill (and recover) things FAST, so you grind very well in crowded areas. There are five quests of note here, most of them starting in Steamwheedle Port (NE of Gadgetzan). There's a pirate hats quest, two WANTED quests worth doing, a grind quest, a ship schedule quest, etc. The map fragments quest and the Find OOX-117 quests are NOT worth doing, by the way. You'll get decent Mageweave cloth here. Grind both inside the fenced area and outside it, just wherever there are respawns. Work your way around.

When: L48, L49, L50
Where: S.W. Tanaris (Thistleshrub Valley)
Mobs: L47-50 Thistleshrubs
Comments: More elementals (so watch that nature-damage wand). These are great for alchemists since they drop herbs such as Gromsblood, Blindweed, Ghost Mushrooms, etc. The Rootshapers go down VERY fast for mobs at this level (low 2000s HP). There are two quests here (both in Gadget, one is for Noggenfogger). I experienced a high green droprate here and even found a purple (Flurry Axe sold for 300g). You should be hitting ~33,000 EXP/hour here.

When: L51, L52
Where: S.W. Western Plaguelands (Sorrow Hill)
Mobs: L50-52 Skeletons/Ghouls
Comments: This is very tricky grinding. This area will be very crowded, so you may not want to grind here at all. For these levels, I sometimes logged on late at night and did these when the server population was low. This area is very streaky. For a few hours one night at like 7pm, I grinded totally undisturbed. It is a great area since the mobs are weak, you get Runecloth, Scourgestones for Argent Dawn rep (keep that A.D. Commission equipped!), etc. Right near a graveyard if you die. You also might want to consider grinding in Felwood for Timbermaw rep or even for Felcloth during these levels, but that will probably be just as crowded.

When: L52, L53, L54
Where: Un'Goro
What: Quests, L52-L54 Tar Lords/Lurkers
Comments: I held off on Un'Goro until L53 for a reason. On my last character, I quested in Un'Goro starting at L50, but I found I was unable to do half the quests and by the time I reached L54-56, I was dry on quests everywhere. The Tar Lords/Lurkers are EXCELLENT to grind since they drop Living Essences (which you can either use for Timbermaw recipes, Hide of the Wild, or just the auction house). These elementals also drop herbs. This spot's very near the camp (vendor/repair). It is important to note, however, that you should begin doing quests again. Do all of the quests in Un'Goro with few exceptions. You probably don't want to do the entire Linken Chain since it involves a lot of travelling, but you can do the first few parts before giving up on the chain. Be wary of the Pterodax quests. Don't spend a lot of time on them. Try to keep an eye out for Pterodaxes while you're riding around like an idiot collecting soil (for a quest in Darnassus, if you're ever near there) and crystals.

When: L55
Where: Felwood
What: Quests, L48-50 Furbolgs/L51-L52 Satyrs
Comments: Felwood is a tricky zone, but many of the quests here are worth the travel if you do and turn in several at once. While crisscrossing across Felwood, I completed the Timbermaw Ally quest several times. You will want at least Unfriendly rep so that you can access Winterspring. The Satyrs drop Felcloth, but these areas will be crowded with farmers. In my experience, the northern area (Jadefire Run) was very crowded, but the southern area wasn't. You can probably get 3/4 of a level out of Felwood. Note that the quest where you have to explore the craters and kill 2 Entropy Beasts (or whatever) is incorrectly marked elite (you can solo it just fine).

When: L55, L56
Where: Winterspring
What: Quests, L53-56 Winterfall furbolgs, L55-58 Yetis
Comments: Felwood and Winterspring go hand-in-hand, and you'll run back and forth between them quite a few times. When you reach WS the first time, you should be carrying two quests from Un'Goro that direct you to Donova Snowden. Most of the quests here are easy and straightforward. The Yeti-Fur one is a bit difficult, so try not to focus on it in particular. Try to do it gradually as you ride around Winterspring. If you can catch the furbolg areas free in the western part of the zone, you can grind there for easy EXP and rep (the Totemics are very weak). The Yetis in the eastern regions and caves are great grinding as well. Note that for the Winterfall Runner quest, there is one Winterfall Runner in Deadwood Village in Felwood that you can kill to obtain the package (so you don't have to run around like an idiot searching for those three runners). Also, the Recipe: Mooncloth pattern can be bought from a vendor in Everlook.

When: L56, L57
Where: Western Plaguelands
What: Quests
Comments: WPL is most definitely my favorite zone in the game. There are so many quests here in such a small zone. Do all four of the cauldron quests. Try to get a group for Araj (I ninjalooted the quest item after a Horde group killed him, actually). At L56, I was able to do the "All Along The Watchtowers" quest without aggroing the elite inside the towers (inch up to the door slowly). There are a couple quests that start in the Inn in Andorhall. At the first farm, there is a ghost inside the house that starts a line of quests. At the second farm, there is a diary on the floor in the barn that starts a line of quests. At the third farm, there is a dead druid in one of the houses that starts a line of quests. Additionally, there is a lone human in the northern part of this zone that starts a line of quests dealing with the Scarlet Crusaders. The final quest in this line is my absolute favorite quest in all of WoW. Tip on soloing it-- PW:Fort, Inner Fire, PW:S, jump on your mount, ride like hell through Hearthglen, use Psychic Scream at the bottom of the tower where the two Paladins are, renew/shield yourself going up the tower, make sure you get credit for the quest at the top of the tower, jump off so that you land on the wall beneath and don't take as much damage, use a potion/Desperate Prayer if applicable, run like hell out of Hearthglen. 6200 EXP and 1.75g quest reward. I was averaging about 50,000-53,000 EXP/hour during this time.

When: L57, L58
Where: Eastern Plaguelands
What: Quests
Comments: Tirion's quests are nice and lead to the eventual "Of Family" quest in Scarlet Strat (which gives a VERY nice cape). They are grueling and take a while, but they are well worth the EXP and the latter quests in the line. Be very careful of the quest "Of Forgotten Memories." It is very unlikely that you can solo it. Slowly make your way toward Light Chappel's Hill, killing all the Bats/Hounds/Grubs and doing a couple Darrowshire quests. There are a lot of Bats concentrated near the Marris Steed, and a lot of Grubs N.E. of that tower near there. Avoid the quest where you have to explode the eight Ziggurauts (takes too long). There are some frustrating quests here where you have to gather a list of supplies or bring something from another zone (e.g., the Fragments of the Past quest where you need Crystal Restore from the Northern Pylon in Un'Goro and a quest item from outside Dire Maul). You'll probably want to avoid these quests. You'll dip down to ~40,000 EXP/hour here, but that's still a lot.

When: L58, 59
Where: Silithus
What: Quests, L57-58 Rock Elementals, L58-60 Twilight Cultists
Comments: Find a group to do all of the elite Hive quests at once, and you'll get nearly half a level upon completion of all of the quests. You'll be grouping a lot in Silithus. You can grind Scorpids for Scorpid Blood drops if you want, or the Rock Elementals for Essences of Earth. The Twilight Cultist camps will probably be crowded with people farming gear/tablets, but then again this entire zone is crowded (albeit, very fun!).

In conclusion:

If you plan on healing in endgame raids, you'll probably want some variation of Disc/Holy (31/20 or 26/25 seem to be the two most popular versions). If you want to respec, you can do this at any time after about L56, but I recommend just rushing through to L60 while you're still Shadow. Also, you might want to PVP your way up to Sergeant for that additional 10% discount before you spec healbot. Of course, you don't have to spec healbot at all. Spec whatever you want.

I'm currently sitting on a L57 Shadowpriest with 650g in the bank account and just over 7 days /played, and I've had a lot of fun thus far on my Priest.

If you have any advice to offer to this guide in any way (equipment to look out for, strategies, mobs/area to grind, etc.) then feel free to post. Thanks for your time.