Copied from US forums post by Haunted:
Most of these have been confirmed to be good, but in order to prevent overcrowding in areas I will try to list several different places, all of which should be pretty decent. More often than not, the areas will only hold one or two people maximum as far as spawn rates go – any more than that and you have too many people and too little mobs to support them. Unfortunately, because of this, you might end up somewhere that isn’t quite as good as you might’ve hoped, and if that’s the case, I apologize in advance.
The levels are approximate, you may or may not have to supplement grinding for one or two levels in order to move onto the next zones early on, although most of the later parts deal mostly with areas to grind in. Also, when a level is listed next to a grind area, that means this is the level at which you can more than likely *start* grinding the mobs. You can probably *stop* grinding them about 4-5 levels past where the start level is. In fact, it’s probably recommended that you do stop 4 levels past them.
Also, keep in mind that this is mostly for Alliance players, as it’s the only faction I’ve played and thus have no experience with Horde lands. You’ll also note the lack of Kalimdor lands up until Feralas/Tanaris. While I’m not saying Ashenvale, Stonetalon Mountains, Thousand Needles or Desolace are bad places, I honestly have no experience with them either, and these areas that I do list are typically sufficient to take you up in levels.
Please note that the level listed beside a place, if there is one, is the level recommended to *begin* the place. Each place should typically be able to last you for 4-5 levels (with the exception of quest areas), whether you have the patience to stay at that one place for that long is another thing.
Thanks to everyone that has contributed and added to the list!
– Newbie Zone :: You may have to leave at level 11.
– Westfall [Sentinel Hill, Westfall Lighthouse]
– Loch Modan [Thelsamar, Farstrider’s Lodge, Stonewrought Dam]
– Darkshore [Auberdine]
– Elite Area: Mo’grosh mound (Loch Modan)
– Instance: Deadmines
Complete the easiest quests in the first area that you go to (Night Elves should visit Darkshore first, Dwarves/Gnomes should visit Loch Modan first, Humans should visit Westfall first.) Move onto the next zone, and do the same, but this time you can complete more of the zone. Leave the most difficult, unless you can get a group for them – don’t waste too much time on it, though. Move onto the last zone and complete all, or nearly all of the quests available in that zone. By this time, you should be about level 17-18. Go back to the first zone, and complete the rest of the quests there.
Typically, my "first zone" is Westfall, in which case you can go back when you are done with both Loch Modan and Darkshore, and finish the Van Cleef line of quests and be at a good level to run the instance.
– Redridge Mountains [Lakeshire, Tower of Azora in Elwynn]
Again, do the easy quests in Redridge, which should take you up to level 21 or 22. Skip the more difficult quests unless you have a group for them. I’d skip the entire Stonewatch Keep area of quests just simply because that entire area itself is crap.
– Redridge Mountains [Lakeshire, Tower of Azora in Elwynn]
– Duskwood [Darkshire, Sven’s Camp, Abercrombie]
– Wetlands [Menethil Harbor, Greenwarden, N. Loch Modan (The Algaz Gauntlet)]
– Ashenvale [Astranaar ?]
– Stonetalon Mountains [?]
– Elite Area: Dun Modr (Wetlands)
– Instance: The Stockades
– Instance: Shadowfang Keep
– Instance: Blackfathom Deeps
– Happy Loot: Naraxis’ Fang (Duskwood)
– Happy Loot: Tiny Crimson Whelpling (Wetlands)
– Grind Area : Dragonmaw Encampment (Wetlands)
– Grind Area : Whelgar’s Excavation Site (Wetlands)
– Grind Area : Rotting Orchard (Duskwood)
– Grind Area : Raven Hill Cemetary (Duskwood)
– Grind Area : Vul’Gol Ogre Mound (Duskwood)
Preparation: Purchase Bronze Tube.
Start at either Duskwood or Wetlands. This is where "quest stacking" can come in really handy if you want to be efficient. There are many, many quests that send you to the same areas, and if you get all of the quests at once, you can knock all of them out without having to run back and forth. Considering Wetlands and Duskwood are fairly bad for having their towns on one side of the map and the quest mobs/areas on the other side, this saves a lot of time. A lot of these are parts of quest chains, so honestly it’s hard to know if you don’t have prior knowledge or do a lot of quest research beforehand – so don’t worry about it too much, but try to be as efficient as you can. Once you finish one of the two new areas, you should be about level 25 and can go finish the harder parts of Redridge, and then finish the last zone.
Note for Stockades : There are 6 quests you can obtain for this instance. Lakeshire, Darkshire, Wetlands, Van Cleef note continuation (Bazil Thredd), and two from Stormwind.
– Stranglethorn Vale [Rebel Camp, Nesingwary’s Expedition, Booty Bay]
– Hillsbrad Foothills [Southshore]
– Arathi Highlands [Refuge Pointe]
– Desolace 
– Thousand Needles 
– Instance: Gnomeregan
– Happy Loot: Howling Blade (Skhowl, Alterac Mountains)
– Happy Loot: Toxic Revenger (Gnomeregan)
– Happy Loot: Tiny Emerald Whelpling (Swamp of Sorrows)
– Grind Area : Kurzen Encampment (Stranglethorn Vale)
– Grind Area : Stranglethorn Raptors (Stranglethorn Vale)
– Grind Area : Growless Cave (Alterac Mountains)
– Grind Area : Gallow’s Corner (Crushridge Ogres – Altarac Mountains)
– Grind Area : Witherbark Village (Arathi Mountains)
– Grind Area : Venture Co. Base Camp (Lake Nazferiti – Stranglethorn Vale)
Preparation: Purchase all 15 pages of The Green Hills of Stranglethorn, 4 Bloodstone Ore.
Start at Stranglethorn Vale. Quest stacking here is also godly, since you don’t want to have to run all the way to the northern end of the zone, do one or two quests and turn them back in at Booty Bay. Rebel Camp’s quests are largely independent of the rest, though, so you can do those first. Then, head to Booty Bay, where you can pick up two main quests for the northern area: Investigate the Camp and the start of the Excelsior chain, both of which takes you to Nesingwary’s Expedition, where you obtain all of the nice happy quests there.
Hillsbrad is only good starting at about level 33, at which point you can actually *do* all of the quests that you could obtain at level 30… Arathi only has one or two quests that you can actually do right now, but it’s listed anyways.
Kurzen Cave has been fixed, which perhaps means that Jungle Fighters and Headshrinkers have as well, so someone test that out please.
– Stranglethorn Vale [Booty Bay, Nesingwary’s Expedition, Rebel Camp]
– Arathi Mountains [Refuge Pointe, Faldir’s Cove]
– Badlands [Various camps around the zone]
– Desolace 
– Elite Area: Stromgarde
– Elite Area: Mosh’ogg Mound
– Instance: Scarlet Monastery
– Instance: Uldaman
– Instance: Razorfen Downs
– Happy Loot: Tiny Black Whelpling (Badlands)
– Grind Area : Lashtail Raptors (Stranglethorn Vale)
– Grind Area : Water Elementals (Stranglethorn Vale)
– Grind Area : The spider cave north of Brackenwall (Dustwallow Marsh)
– Grind Area : Rock Elementals (Badlands)
– Grind Area : Elder Crag Coyotes (Badlands)
– Grind Area : Scalding Whelps (Badlands)
– Grind Area : Agmond’s End (Badlands)
– Grind Area : Ziata’jai Ruins (Venture Co. Stuff, Stranglethorn Vale)
– Grind Area : Naga Explorers (Stranglethorn Vale)
– Grind Area [4X]: Grimtotem Compound (Feralas – Low 40s, not absolutely sure)
Preparation: Purchase 9 Blue Pearls, 1 Frost Oil, 1 Gyrochronatom, 4 Buzzard Wings, 1 Elixir of Water Breathing.
37 is where you obtain your next batch of quests from Stranglethorn Vale. Arathi Highlands has fairly decent quests in Faldir’s Cove, or if you’re feeling group-friendly, knock out the ones in Stromgarde. Badlands has a lot of nice quests, but you have to actually find them. This is where supplementing experience by grinding becomes more efficient at times, and at 42-43 you should be closing the last of your Stranglethorn quests, and be finished with Badlands and Arathi both, as well.
Scarlet Monastery. Black Menace or Sword of Serenity. Nothing more needs to be said – except where to obtain the quest: Desolace.
– Feralas [Feathermoon Stronghold]
– Tanaris [Gadgetzan, Steamwheedle Port]
– Instance: Zul’Farrak
– Grind Area : Wastewander Bandits (Tanaris)
– Grind Area : Isle of Dread (Feralas – Inside Cave; South of Feathermoon Stronghold)
– Grind Area : Feral Scar Vale (Feralas)
– Happy Loot: 14slot bag! Cortello’s Riddle quest, starts in one of the ships in Stranglethorn Vale for the end of the Bloodsail Buccaneers line of quests.
Fairly self explanatory, I guess. Not much going on here, really.
– Tanaris [Gadgetzan, Steamwheedle Port]
– The Hinterlands [Aerie Peak]
– Instance: Zul’Farrak
– Instance: Maraudon 
– Instance: Temple of Atal’Hakkar 
– Grind Area : Undead in Western Ashzara (Azshara)
– Grind Area : Dunemaul Compound (Tanaris)
– Grind Area : Elementals/Golems around The Cauldron + N of it (Searing Gorge) – Elemental Leatherworking Supplies
At this point, you more than likely have a bunch of quests saved up for The Hinterlands – including but not limited to, the quest to gather Wildkin Feathers (from Rutheran Village), the quest to obtain a snapshot of Gammerita, the quest to obtain Violet Trajan, the last part of Cortello’s Riddle, etc. Feel free to knock them all out at once.
– Searing Gorge [Kalaran Windblade]
– Blasted Lands [Blood Elf camp]
– Un’Goro Crater [Marshal’s Refuge]
– Felwood [Emerald Sanctuary]
– Burning Steppes [Morgan’s Vigil]
– Instance: Temple of Atal’Hakkar
– Grind Area : Deadwood Village (Felwood)
– Grind Area : Ruins of Eldarath (Azshara)
– Grind Area : Dreadmaul Rock (Firegut Ogres – Burning Steppes)
– Grind Area : Legash Encampment, Thalassian Base Camp, Ursolan (Azshara)
– Grind Area : Jaedenar (Felwood)
– Grind Area : Sorrow Hill (Western Plaguelands)
– Grind Area : Ironwood Trees in northern Felwood
– Grind Area : Temple of Arkkoran (Azshara)
– Grind Area : Jadefire Run, Felpaw Village (Felwood)
The quests in Blasted Lands all stack on top of each other. Five in total, one for each stat; they give 4700 experience per turn-in, along with always the chance of obtaining a Draenethyst Sphere for Kumi’sha the Collector with every mob you kill. Un’Goro Crater has the biggest collection of quests out of all of these zones.
– Winterspring [Everlook, Starfall Village]
– Western Plaguelands [Chillwind Point]
– Eastern Plaguelands [Light’s Hope Chapel]
– Instance: Blackrock Depths
– Instance: Blackrock Spire 
– Instance: Stratholme 
– Instance: Scholomance 
– Grind Area : Felstone Field, Dalson’s Tears, The Writhing Haunt, Gahrron’s Withering (Western Plaguelands)
– Grind Area : Northridge Lumber Camp (Western Plaguelands)
– Grind Area : Corin’s Crossing (Eastern Plaguelands)
– Grind Area : The Pillar of Ash (Burning Steppes)
– Grind Area : Winterfall Village (Winterspring)
– Grind Area : Blackrock Stronghold (Burning Steppes) [NOTE]
– Grind Area : Ice Thistle Hills (Wintespring) – Rugged Leathers
Notes: Blackrock Stronghold is probably the best place to grind for a rogue; however, you do have to get used to how the roamers work. You’ll probably die a few times along the way while learning them. Also, this might likely be another area where they’re increasing the armor on the mobs, but two of the four kinds of orcs that spawn are casters, anyway. Ice Thistle Hills is not the best place to grind for experience, but it’s probably one of the better places to get a nice quiet grind going and also leave with a few backpacks full of Rugged Leather.
Okay — So this isn’t really a "complete" grind area list, because honestly, you only have to sit down and grind at one or two places for a few levels before you can move onto the next tier of zones, and that’s typically what I did. Just sit down and grind at one place, don’t move around everywhere and you’ll be out of there in a few hours or days. I’ve tried my best to list the best quest areas and zones as well for those that would rather try to quest and not grind their way through the game, too, but that’s also somewhat incomplete, especially at later stages since I don’t have much knowledge of the various new quests they implemented in Winterspring and the Plaguelands.
Anyway, hope it helps to answer some of the "where can I go at level X?" questions.
The promised Horde section. Implemented from various posts in this thread by Horde players that have contributed. Some of these spots were also taken from Ratama of Nurfed.
14-16: Pirates south of Ratchet (Barrens)
16-19: Various wildlife, especially cats, in Barrens. Note that if you’re using a dagger crit build, then exp for this is insane.
19-22: Bramblescar (Barrens) on lizards and quillboars. Unless you’re doing leatherworking, just leave the lizards alone.
22-24: Tarren Mill (Hillsbrad) on bears and spiders. Dagger crit chain heaven.
24-26: Across the road from Tarren Mill on tougher bears and cats. Dagger crit chain heaven.
26-28: Spiders/Bears in back of Syndicate tower (Hillsbrad)
28-30: Cats and Spiders around Nethander Stead (Hillsbrad)
30-32: Harpies inside cave (Thousand Needles)
NE of Brackenwall (2 min run there)- Raptor den
36-37 raptors there – lots of them
40-42 Skeletons in Desolace. They’re about the only squishies I could find to grind on. It’s part of an Alliance quest, so the usual PvP warnings apply.
42-45 Jungle Stalkers in SV. The loot drop isn’t nearly as good as the Lashtails, but they’re as squishy as ever. Also, they don’t seem to have the knockdown attack that the lashtails do. Virtually no one kills these guys on my server, and they’re easily chainable.