I don't know how to admit this to you guys, given my lengthy advocacy of the Holy tree, but I seem to be... gulp.. a Disc priest.
I have dual spec and was using Disc for battlegrounds and Holy for leveling (with a warrior, and just because it's fun). But after a few deaths in the Outland after which I said, "You know, we'd have lived through that if I was Disc" I just said f'it and went Disc for my leveling spec as well.
So. Additional consideration on all these leveling specs we've been talking about lately: whether or not you're leveling on a PVP server. Say what you want about the viability of other specs for PVP, and yes, given the right gear/skill/talents at endgame, they really are just dandy. But Disc is really The Thing That Lets You Live.
At 80 I will still probably change to Holy for my PVE spec, just for fun and variety. But then again, maybe not. By then I'll have the Disc buttons all memorized, you know?