Summary:
Do very, very few quests from level 30 to level 52. Then do quests as quickly as you possibly can until you ding at 60.
Levels: 21, 22, 23, 24
Where: Southeast Duskwood (Tranquil Gardens Cemetary)
Mobs: Level 21-23 Skeleton Warriors/Mages
Comments: The area is packed with mobs and has a fast respawn rate. Should get around 18,000 EXP/hour here. Do the quest titled 'The Night Watch' because it is a really good grinding quest.
Levels: 24, 25, 26
Where: Northwest Duskwood (Raven Hill Cemetary)
Mobs: Level 23-25 Skeletal Fiends/Horrors
Comments: If this area is packed with players, 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; don't die as you will have a long walk.
Levels: 26, 27, 28, 29
Where: Duskwood
Mobs: Level 27-29 Ghouls (Raven Hill Cemetary/Catacombs), Level 26-30 Ogres (Ogre Mound)
Comments: I'm sorta at a loss about what to say for these levels. The Worgen quest gives a 24.2 dps wand (if you didn't do 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 travel time. Set your hearth to Darkshire, also.
Levels: 29, 30, 31, 32
Where: Northeast Duskwood (Manor Mismantle)
Mobs: Level 29-30 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; stay away from 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.
Levels: 33, 34
Where: East Desolace (Kolkar Village)
Mobs: Level 31-34 Centaurs
Comments: There is one grind quest here for Centaur Ears, and the quest reward is a 7 SPR ring. You can easily get a ton of silk and mageweave here. Level 33-36 were the slowest leveling paces and the 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 Stranglethorn Vale, especially if you are on a PvP server as you will not get anything done quickly.
Levels: 35, 36
Where: Southeast Desolace (Magram Village)
Mobs: Level 32-36 Centaurs
Comments: This can be a tough area to grind in, if you let it be. You have to find the correct spot and just get in a groove. 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, and you can cross the bridge and kill the mobs on the island if you would like. Beware and avoid the Pack Leaders at all costs (they spell easy death). Figure out their movement paths so that you can avoid them.
Levels: 37, 38, 39, 40
Where: Northwest Badlands (Dustbowl)
Mobs: Level 37-39 Lesser Rock Elementals
Comments: Your greatest grinding will probably take place here. Your mount money will come from here. Elemental Earths sell for 75 silver to 1 gold a piece on the auction house on my server, and stacks of 20 Solid Stone sell for 1.5 gold to 2 gold. Make use of the auction house! At level 40, I had about 200 gold 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 (level 175).
Levels: 39, 40, 41, 42
Where: South Badlands (Mirage Flats)
Mobs: Level 39-40 Rock Elementals
Comments: These guys are located very south in the mountain 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 won't have much mageweave (if you're a tailor). At level 40, your EXP/hour will shoot up to around 30,000!
Levels: 42, 43, 44, 45
Where: Southwest Badlands (Camp Something)
Mobs: Level 42-44 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 best for grinding. The area is circular, so you can just go around in circles over and over again. If the eastern area becomes packed with players, you can switch back and forth between the two areas easily. Also, there is another grinding quest here.
Levels: 45, 46, 47
Where: East Tanaris (Lost Rigger Cove)
Mobs: Level 44-45 Southsea Pirates
Comments: After 9 levels in Badlands, you finally get some change of scenery. This area can be have quite a few players around, 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 (Northeast 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.
Levels: 48, 49, 50
Where: Southwest Tanaris (Thistleshrub Valley)
Mobs: Level 47-50 Thistleshrubs
Comments: More elementals (so don't use that nature-damage wand). These are great for alchemists since they drop Gromsblood, Blindweed, Ghost Mushrooms, etc. The Rootshapers go down VERY quickly for mobs at this level (low 2000s HP). There are two quests here (both in Gadgetzen, one is for Noggenfogger). I experienced a high green drop rate here and even found a purple (Flurry Axe sold for 300 gold). You should be hitting 33,000 EXP/hour here.
Levels: 51, 52
Where: Southwest Western Plaguelands (Sorrow Hill)
Mobs: Level 50-52 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 reputation (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 reputation or even for Felcloth during these levels, but that will probably be just as crowded.
Levels: 52, 53, 54
Where: Un'Goro
What: Quests, Level 52-54 Tar Lords/Lurkers
Comments: I held off on Un'Goro until level 53 for a reason. On my last character, I quested in Un'Goro starting at level 50, but I found I was unable to do half the quests and by the time I reached level 54-56, 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 drop herbs also. 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.
Level: 55
Where: Felwood
What: Quests, Level 48-50 Furbolgs/Level 51-52 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 reputation 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).
Levels: 55, 56
Where: Winterspring
What: Quests, Level 53-56 Winterfall Furbolgs, Level 55-58 Yetis
Comments: Felwood and Winterspring go hand-in-hand, and you'll run back and forth between them a lot. When you reach Winterspring the first time, you should have two quests from Un'Goro that direct you to Donova Snowden. Most of the quests here are pretty 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 reputation (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 searching for those three runners). Also, the Recipe: Mooncloth pattern can be bought from a vendor in Everlook.
Levels: 56, 57
Where: Western Plaguelands
What: Quests
Comments: Western Plaguelands is most definitely an awesome 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. At level 56, 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 probably one of the best quests in World of Warcraft. 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 1.75 gold quest reward. I was averaging about 50,000-53,000 EXP/hour during this time.
Levels: 57, 58
Where: Eastern Plaguelands
What: Quests
Comments: Tirion's quests are nice and lead to the eventual 'Of Family' quest in Scarlet Strat (which gives an AWESOME 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 Northeast 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 pretty good.
Levels: 58, 59, 60
Where: Silithus
What: Quests, Level 57-58 Rock Elementals, Level 58-60 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 packed but very fun!
In conclusion:
If you plan on healing in end game 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 level 56, but I recommend just rushing through to level 60 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.
If you were lucky enough to register a 
