For Mania, the answer turned out to be going to a Death Knight -- she may discover that she's opened up a whole new world in choosing to level with a completely different class. An alt was also the answer for me, but I have a few different reasons, the first of which is Engineering -- I thought for a while, after hitting 450 Leatherworking on my (also Skinning) Hunter that I would switch to Engie and just buy all the mats, but I think it'll be more fun (and profitable) leveling up a Mining/Engineering alt. And he can always make the BoE chopper for any of my characters as well.
But while an alt is sometimes the answer, there are lots of things you can dive into to find a completely new area in the game -- try Arenas for the first time, do a Wintergrasp grind, seek out an old world reputation or title that you've always wanted. We're very lucky, in a game like this, to have lots and lots of new things to try even when it seems like we've done everything we want.