I've been compiling this for a few hours now. It may not be entirely accurate (feel free to make corrections if I missed something), but it's certainly a start if you're looking for a reference on where to level.
PANDARIA:
Townlong Steppes: 24
Dread Wastes: 24
Kun-Lai Summit: 23-24
Valley of the Four Winds: 23-24
Krasarang Wilds: 23-24
Jade Forest: 23-24
Vale of Eternal Blossoms: 23-24
CATACLYSM:
Twilight Highlands: 23
Uldum: 23
Deepholm: 22-23
Vashj'ir: 22-23
Hyjal: 22-23
NORTHREND:
Crystalsong Forest: 22
Storm Peaks: 22
Icecrown: 22
Zul'Drak: 22
Dragonblight: 22
Grizzly Hills: 21
Sholazar: 21
Borean Tundra: 21
Howling Fjord: 21
OUTLAND:
Shadowmoon Valley: 20
Netherstorm: 20
Blade’s Edge Mountains: 19
Nagrand: 18
Zangarmarsh: 18
Terokkar Forest: 18
Hellfire Peninsula: 17
KALIMDOR:
Winterspring: 17
Silithus: 16
Ahn’Qiraj: 16
Moonglade: 15
Un’goro Crater: 15
Felwood: 14
Tanaris: 13
Thousand Needles: 13
Dustwallow Marsh: 12
Feralas: 11
Southern Barrens: 9
Desolace: 7-8
Stonetalon Mountains: 5-6
Ashenvale: 4-5
Azshara: 3-5
Darkshore: 3-5
Bloodmyst Isle: 3-5
Northern Barrens: 3
Mulgore: 1
Durotar: 1
Azuremyst Isle: 1
Teldrassil: 1
EASTERN KINGDOMS:
Blasted Lands: 16
Burning Steppes: 15
Swamp of Sorrows: 14
Searing Gorge: 13
Badlands: 13
Eastern Plaguelands: 12
Hinterlands: 11
Western Plaguelands: 10
Cape of Stranglethorn: 9
Northern Stranglethorn: 7-8
Arathi Highlands: 7
Hillsbrad Foothills: 6
Wetlands: 6
Duskwood: 5-6
Silverpine Forest: 3-5
Ghostlands: 3-5
Loch Modan: 3-5
Redridge Mountains: 3-5
Westfall: 3
Tirisfal Glades: 1
Eversong Woods: 1
Dun Morogh: 1
Elwynn Forest: 1