The game has single player, and it's great fun. It's a psuedo-third person FPS/first person (depending on what weapons you use and what view you prefer) action-rpg.
The only downside to it is that the random level generator is truly boring. This does make the designed bits EXTREMELY good.
Edit: The stick to WoW option which I just saw makes me want to remind you that it's not an MMORPG at all. It's an action-rpg which had an optional pay-to-play online feature. By optional I mean you could play without paying but you didn't get any of the extra stuff. This is what made it fail.