It becomes an issue when you don't have infinite resources. And the longer the game gets, the increase in cost goes up exponentially.
Well, according to some (Kalfear), WoW is a poor knockoff of implementations that were copied from other games and all that.
To be honest though, as much as I'm a fan of WoW, it really isn't that deep of a game. Quests and all that are very similar, and don't really affect the gameworld at all.