WoW runs fine on shitty computers. That's the edge it has over all these other, more esoteric MMORPGS. Guild Wars was a nice idea: pay once for the game, no monthly fee, easy levels, and lots of expansions. The only problem was that it's minimum requirements were head-and-shoulders above WoW's. Warhammer Online requires a goddamn steroid machine to run at minimum specs, which is why WoW is more popular.
The vast majority of people who play computer games don't invest enough money in them to be able to play high-end stuff. I happen to be one of those people, unfortunately. If my computer could run WHO, I'd drop WoW in less than a heartbeat.