Although you can level to well over level 70 in Outland you should consider moving to Wotlk zones at around level 67 – 68 for MUCH better XP. The XP here is better and questing and leveling is a lot quicker.
Howling Fjord Level 68-72
Along with Borean Tundra, the Howling Fjord is the starting zone in Northrend. Both zones are viable from around level 68 and combining questing in both zones can be very efficient.
Borean Tundra Level 68 – 72
Borean Tundra is one of the two starting zones of Wotlk and is one of the first areas you will visit. Go here at around level 68, then move on at around 72.
Dragonblight Level 71-74
The Dragon graveyard of Dragonblight is a big zone in the middle of Northrend. There are quite a few quest hubs here and a lot of quests to do. Hit up this zone after finishing up in Borean Tundra and Howling Fjord.
Grizzly Hills Level 73-75
After finishing up in Dragonblight move on to Grizzly Hills. Tons of quests and good places to level.
Zul’Drak Level 77-78
At around level 77 it’s a good time to hit up Zul’Drak. Mostly inhabited by Trolls, this zone has plenty of quests and areas to level in for both factions.
Sholazar Basin Level 78-80
The Sholazar Basin is a dense jungle. The quest here should take you to level 80, but if you still have a ways to go you can combine leveling here with some of the zones below.
Icecrown Level 77-80
Has a few good leveling locations with some quests.
Storm Peaks Level 77-80
Also a viable leveling location for your late 70’s with some good quests.
Utgarde Keep Level 67-68
The Nexus Level 68
Ahn’kahet: The Old Kingdom Level 70
Azjol-Nerub Level 69
Drak’Tharon Keep Level 71
The Violet Hold Level 72
Gundrak Level 73
Halls of Stone Level 74
Halls of Lightning Level 75
The Oculus Level 75
Caverns of Time: Culling of Stratholme Level 75
Utgarde Pinnacle Level 75
Trial of the Champion Level 75
Forge of Souls Level 75
Pit of Saron Level 75
Halls of Reflection Level 75