Aside from all of the technological errors like having too few servers at launch to keep up with the demand, Pardo thinks that the single greatest mistake as far as design of the game is concerned was the choice to introduce competitive "e-sports" into the game via Arenas.
"I'm not sure that that was the right thing to do with the game ... we just added it on, so it continues to be very difficult to balance. Is WoW a PvE cooperative game, or a competitive PvP game? ... I don't think we ever foresaw how much tuning and tweaking we'd have to do to balance it in that direction. Either I'd go back in time to before WoW ever shipped and change the rules to make the basic game more conductive for being an e-sport, or if not that, just say it doesn't make sense. Right now, WoW has a bit of a schizophrenic philosophy behind it, and we're trying to figure out how to guide it."