I am still pretty into WoW to be honest. I will probably try out Diablo when it comes out though to get a feel for it.
The thing with WoW for me was that I'm not going to spend 10+$ for the future, which I don't know how will turn out. In the start I was all over the game, so I guess those were kinda well spent 10$ (gaming wise, not life wise :laugh), but in the end, I got a bit tired of it and didn't have that much time to play it, and there were even weeks I played barely 2h or so.
Diablo will be awesome though, because I haven't heard about a monthly fee or anything like that... yet.