I had a leveling streak with Myde on my weekend. She started in Borean Tundra and did questing through it all, except for the DEHTA and murloc hubs.
And she leveled… from 58 to 71.
The core source of experience was actually dungeons. Every dungeon brings you up for 0,7-1 level, and they are lots. Every next level opens up 2-4 more from both Outland and Northrend. I go without repeating a single one of them, and I still have at least 12 to go.
After finishing the Tundra (Coldarra was obligatory, because Myde is arcane), I just stopped bothering with questing and went on a tour opening Northrend map. Every subzone ticks by 1000+ experience, so you could easily gather at least 1-2 levels merely by opening the map. Thus you have your Northrend opened while waiting for dungeons to pop up.
It’s super fast, and I’m planning to get her to 80 just by exploration and dungeons. If I have some time left from opening Outland, I would quest in Nether zone of Outland a bit – a largely ignored quest spot previously for obvious reasons.
We shall see about Pandaria and Cataclysm, but the speed of leveling up in Northrend/Outland is just amazing. In few hours she left other allied toons 10 levels behind. The same plan goes with the other allied toons: exploring while waiting for dungeons, and then grabbing a zone of choice to quest a bit – they are all available at 67, be it Shadowmoon Valley, Storm Peaks or Icecrown.