At around level 58 you should start considering leaving Kalimdor and the Eastern Kingdoms behind and move on to Outland. You can still get some decent XP, but mobs in Outland gives almost double the XP compared to old world zones. So when you are ready head over to the Blasted Lands and go through the dark portal. At around level 67 – 68 you can and should move on to Wotlk zones as they provide much better XP.
Hellfire Peninsula Level 58-63
Hellfire Peninsula is the first zone you get to when stepping through the dark portal. Both the Alliance and the Horde have several settlements and quest hubs in the zone. Leveling is quick here, so stay until around level 62-63 then move on to Zangarmarsh.
Zangarmarsh Level 60-64
The next zone after Hellfire Peninsula is the Mushroom forest of Zangarmarsh to the west. Like all of Outland it has plenty of quests for both factions and leveling is generally quick and easy.
Terokkar Forest Level 62-65
Terokkar is a viable option for players finishing up in Zangarmarsh. Most of the time I tend to skip this zone and go straight to Nagrand at around level 63 – 64.
Nagrand Level 64-67
After Zangarmarsh you should move on to Nagrand. The zone have quite a few good quests, and if your zone holds Halaa you will get some advantages like more quest givers and turn ins. The zone is good leveling until around level 66 – 67.
Blade’s Edge Mountains Level 65-68
Blade’s Edge Mountains provided some good XP and leveling for both factions. But I usually head over to Wrath of The Lich King zones at around level 67 – 68.
Netherstorm Level 67-70
Netherstorm is a good zone for leveling but consider moving to a Wotlk zone before finishing up and leveling all the way to 70 here. The same goes for Shadowmoon Valley below.
Shadowmoon Valley Level 67-70
Hellfire Citadel: Hellfire Ramparts Level 57-67
Hellfire Citadel: The Blood Furnace Level 59-68
Coilfang Reservoir: Slave Pens Level 60-69
Coilfang Reservoir: The Underbog Level 61-70
Auchindoun: Mana-Tombs Level 62-71
Auchindoun: Auchenai Crypts Level 63-72
Caverns of Time: Escape from Durnholde Keep Level 64-73
Auchindoun: Sethekk Halls Level 65-73
Coilfang Reservoir: The Steamvault Level 67-75
Auchindoun: Shadow Labyrinth Level 67-75
Hellfire Citadel: Shattered Halls Level 67-75
Tempest Keep: The Mechanar Level 67-75
Tempest Keep: The Botanica Level 67-75
Caverns of Time: Opening the Dark Portal Level 68-75
Tempest Keep: The Arcatraz Level 68-75
Magisters’ Terrace Level 68-75