Try listing other MMORPGs as comparison instead of braindead single player first person shooters.
More players = higher CPU requirements
It's not a limitation of WoW's engine but a limitation in forumgoer's brain to realize they're doing apples to oranges comparison.
You're comparing single player game (Mario Kart) to an MMORPG with hundreds of players in single zone. WoW's framerate or lack of it has got nothing to do with graphics engine but the part which handles the simultaneous actions of all players around you. Or to dumbify it even more, everybody knows that addons that deal with combat log (Recount, Deathnote etc) makes the game slower.
It's the combat log (ie. player actions) which is the slowest part of WoW, not the renderer.