While I very much agree with Miyamoto that "a delayed game is eventually good, but a rushed game is forever bad," the reason listed for delaying it just isn't good enough in today's game dev cycle. All delaying it now would do is give Blizz less data with which to fix abilities while giving the #PullTheRipcord community more time to try and pressure the devs to decouple the covenant/class abilities. Perfect balance really is not attainable unless you make each class carbon copies, and trying to balance the abilities right now without a full slate of players showing the devs just how broken things can get is a fool's errand.
It will be far better for WoW to get the expansion out and see how much fun it actually becomes while in the hands of the player base.