Azeroth

Level 01-11: Goblin, Blood Elf, Orc
Level 01-11: Undead, Troll, Tauren
Level 11-20: Azshara
Level 20-25: Ashenvale
Level 25-30: Northern Stranglethorn
Level 30-35: Cape of Stranglethorn
Level 35-40: Dustwallow Marsh
Level 40-44: Feralas
Level 44-48: Thousand Needles
Level 48-51: Tanaris
Level 51-54: Un'Goro Crater
Level 54-55: Swamp of Sorrows
Level 55-58: Blasted Lands

Outland

Level 58-61: Hellfire Peninsula
Level 61-63: Zangarmarsh
Level 63-65: Terokkar Forest
Level 65-67: Nagrand
Level 67-68: Blade's Edge Mountains

Northrend

Level 68-71: Howling Fjord
Level 71-73: Borean Tundra
Level 73-75: Dragonblight
Level 75-77: Grizzly Hills
Level 77-78: Zul'Drak
Level 78-80: Sholazar Basin

Cataclysm

Level 80-82: Mount Hyjal
Level 82-83: Vash'jir
Level 83-84: Deepholm
Level 84-85: Uldum