Hinterlands was the best thing I've found so far and that was with starting it late at 49. I didn't realize how many Kill X of Y quests there were or simple one item collection quests. I did 15 quests. Hit 50 while questing and 51 and 52 during turn ins.

I dinged 53 from turning in each of the robotic chicken rescue quests (Tanaris, Feralas, Hinterlands) and cloth turn ins.

On my way to Winterspring, I found 5 simple quests in Felwood that allowed me to hit 54 and I'm one good quest from 55. A couple of the Felwood quests yield 29k in XP.

I'll be trying Winterspring and EPL based on comments in this thread in hopes of hitting 60 tonight.

This was on Horde btw.

PS. I'd love to see a thread discussing good areas for 20-48. I sort of winged it from past experience on single toons and I feel I wasted a lot of time this first time through.