20+: Tarren Mill(H), Stonetalon Mountains(H)
Kill mobs and quest
25-29: Galak humanoids in Thousand Needles
These guys(and i guess some are girls..) die so fast that my Paladin was able to level extremely fast killing these, even when they were green to me.
30+: Stranglethorn Vale, Tigers/Panthers & Quest. Thousand Needles, Shimmering Flats, Basilisks/Turtles/Birds & Quest
Self-explanitory.
31-34: Yetis - Hillsbrad Foothills
Yeti's in Hillsbrad are kick ass. Also great for skinners who need some free leather.
34-37: Venture Co. Goblins in STV
These mobs are great! They die quick, and their casters. Their lightning bolts tend to hit a bit hard for their level, take caution.
37-39: Murlocs in STV
These guys are also great. Enjoy the sun, the sound of the murlocs, and enjoy that pool because you're gonna be dippin in there for quite a while.
40+: Rock Elementals & Whelps in Badlands
This is one way to go once you hit 40 and get your brand new spankin mount. These mobs will help you get your money back. Rock Elementals drop very nice vendor-trash loot and Whelps have a chance to drop Dragon pets which should sell for a bit in the AH. On my server, they probably don't but Whelps are also easy to kill. There is also quests for rock elementals, consider that a little bonus.
40-44: Wastewander in Tanaris
These are what I usually grind on. Some of these mob gouges you, and theres also a higher level patrol which can make this place a bit irritating. But this place is still a very good grinding place, and you'll get a lot of mageweave. Mageweave on my server usually go for around 3g-5g a stack, usually because you don't see as many Mageweave in the AH, not as much as Runecloth, Netherweave, Silk and Linen. Theres also quests for Wastewanders.
44-49: Dem Filthy Nagaz stole my bike, in Feralas
These mobs are extremely kick ass. They die so fast.. its not even funny. The only thing that I didn't like about this place is the loot, having 30 pearls in your bag can get quite annoying. Other than that, this is probably the #1 leveling spot.
47-49: Dunemaul Ogres in Tanaris
I'll be honest here, this place is great for some Mageweave and Silver drops, even a blue sometimes. I however, did not like this place on Melee characters I leveled with. This place was great for casters though, mainly Shadow Priests. Worth a shot to try these guys, and see if your XP/hr is decent enough.
49-51: Thistleshrubs in Tanaris
This place is also similar to Dunemaul Ogres, a bit easier, but their root totems can get quite annoying. There's also a quest for these guys, for the Noggenfogger quest. I didn't like this place too much with my Paladin, but i loved it on casters.
50+: Un'goro Crater
Welcome to level 50. Time to wander a bit in Un'goro, OR you could come back here at around 52. But other than that, this place is self-explatinory. You can get pretty nice xp, killing mobs and doin all the quests there.
50-54: Skeletons, Zombies in WPL -OR- Sorrow Hill
These mobs are pretty cool. They die fast enough, they drop scourge tokens & bone fragments which can sell for a bit, or can be saved up for Argent Dawn rewards. One level and I got about.. 200 ish Bone fragments and 100 tokens. Also, lots of runecloth here. I got to 300 Firstaid in this area. As you get a few more levels, you can move on from the far west area to the ones a bit more to the east(Dawson Creek or w/e).
53-56: Blood Elves of Ashzara
These are probably #2 on my best leveling spot list. They die super-fast, and they drop fantastic loot. Few cons though, are the fact that this place is crowded with farmers & other levelers(probably other Blood Elves or Draenai.) This place is also quite rough at the lower levels, two or more blood elf mobs can nuke you down pretty quick, they hurt. They also heal, but that isn't such a big problem.
56+: Ice Cave Yeti's of Winterspring
Few Cons/Pros about this place. They die quick, they drop nice loot(and leather for skinners), there are tons of them, and there is a quest for them for small bonus xp. You even get your own minature yeti! Now here's what I don't like bout em, they freeze. As a melee class, I tend to hate being frozen for more than 3 seconds. Few of them also hit a bit hard, I forgot if they enrage or not. This place is better suited for casters imo, but melee should do fine here too.
57-60: Ghosts of Winterspring
This was the final grinding place for my Warrior. These drop nice loot, sometimes enchanting patterns for Unholy(I think) and can get lots of xp from these.
58+: Hellfire Peninsula
This is completely up to you, there are tons of people at 58 goin here. In all honesty, I wouldn't even want to visit this place til im at least 60.
58+: Instance
Last few levels may be somewhat hard. But by this level, you are most likely able to go to Scholo/Strath/DM, or Ramparts even, with a group of friends you know.
58+: Grind in previous locations
By this level, if you don't want to instance, or go to Hellfire just yet, I would recommend goin back to the Blood Elves in Ashzara or Ice Cave Yeti's in Winterspring and rip through shit, they should all be greens after all.