I'm in the middle about the expansion. I play somewhat casually now, still a few hours a day sometimes but I am only playing a level 68 Shaman right now. I'm not too excited about the raised level cap because I don't really enjoy levelling, but it will be fun with new zones I suppose.
Hero classes will be cool, I'm curious what the other hero classes will be. I'm personally really hoping for Blademaster.
I was at Blizzcon and got to try out the expansion, but it was highly disappointing. I came out of the press conference where they spoke about the Death Knight, and being level 80; only to sit down at the demo and can only play my current, level 70 character, in 1 new zone, and only 20 minutes to do so. It was really stupid and pointless. I wish they had the Death Knight ready for us to try, but whatever.
They gave us beta keys, so I'm assuming that it will be for WotLK, so when that comes out I'll probably play it for an hour before I give away my beta account like I did last time.