There are certain differences between a stylized (Quite shoddily made in parts) MMO and a FPS mod - the fact its first person is quite a major one (and WoW's 'first person' really isn't the same thing). The other biggy is that an MMO focuses on making the whole world somewhat believable and entertaining - whereas an FPS has only a single map and its contents at any one time.

And WoW is still a minority of gamers in general