I am a veteran WoW player. Started playing I think a week after it went live. Still play a bit now and then. Love WoW, I will not knock WoW, love that game.
I played some EVE too, about five months way back there. Good game, even worse time suck than WoW, though.
Now GW is not really a MMO even though it sort of feels like one. I have 7 classes full up over there, all three chapters and Eye of the North. GW has been very successful, enough so that GW2 is coming with some absolutely stunning gameplay innovations. How did GW survive and prosper in the shadow of WoW? No monthly fee is how!
Clearly Diablo III is going to be an instant classic, fully as much as Diablo 2 Lord of Destruction. I've played a lot of D2 LoD and I really and truly suck at it! But it is fun. Torchlight is not nearly so nasty, and even much more fun!
Oh, by the way, PlugY is now released for the latest D2 patch 1.13c. Just tested it, smooth as silk!
Most humorous post on Diablo III I've seen lately was that with Blizzard completely focused on StarCraft 2 and getting Cataclysm out and stabilized, "the guys working on Diablo 3 are both very depressed."