I just found out the extremly fast leveling method!
I think you all know the scourge invasion event. In these spawns, there is alot of level 69 and mobs. And now, here comes the method:
You can do two different ways, i think the first one is my efficient but that's your choice.
Well, you take your low level alt to the spawns, and one or two 70's and be in a party and just kill everything, then you will get about 200xp each.
OR, you can be in different parties. And let the low leveled to some damage so it gets tagged to him, then kill it. Then you get about 400xp per mob.
I will get an Screenshot soon.