40: Feralas / Tanaris (feralas sucks questwise but does have some very nice kill quests worth loads of xp)
And The Hinterlands are made jst for multiboxers from lvl 44, on horde side ... I picked up around 10 kill quests, but you do have to be 44 to get the quests. I'm sure it's about the same for alliance.
46, Searing Gorge ? Badlands also has some eaxy xp quests.

Good luck 8) Oooh and any ideas still welcome