Almost any dungeon crawler would probably satiate you for now. Torchlight II should be out
relatively soon, and reminds me a lot of the transition between Diablo and Diablo II (more story, more open areas, etc). You can always play the original Torchlight, but I'd highly recommend checking out some of the old threads on it here, as I would
not recommend playing without mods.
Diablo III should be out early next year as well.
I am surprised nobody has said Borderlands yet. That was just like WoW only in single player format. "Hey go kill 12 boars!" quest crap. I enjoyed it though.
That game can be ridiculously addicting if you also tend to be a fan of dungeon crawlers (like Diablo). The idea of one-upping your loot is such an enticing draw!
Unfortunately, the game just went off sale on Steam! Also, it's really not as much like WoW unless you really liked playing Hunters.