Northrend Leveling Zones

Level 68-72: Howling Fjord or Borean Tundra
Level 71-73: Dragonblight
Level 73-75: Grizzly Hills
Level 74-75: Crystalsong Forest
Level 75-77: Zul’Drak
Level 75-78: Sholazar Basin
Level 76-80: The Storm Peaks
Level 77-80: Icecrown

Northrend Dungeons

Level 70-72: Utgarde Keep (Howling Fjord)
Level 71-73: The Nexus (Borean Tundra)
Level 72-74: Azjol-Nerub (Dragonblight)
Level 73-75: Ahn’Kahet (Dragonblight)
Level 74-76: Drak’Tharon Keep (Grizzly Hills)
Level 75-77: The Violet Hold (Dalaran)
Level 76-78: Gundrak (Zul’Drak)
Level 77-79: Ulduar: Halls of Stone (Stormpeaks)
Level 80: Ulduar: Halls of Lightning (Stormpeaks)
Level 80: The Oculus (Borean Tundra)
Level 80: Caverns of Time: Stratholme
Level 80: Utgarde Pinnacle (Howling Fjord)