Imo people seem to be more upset about them giving the release date at all if they weren't sure they will have the game finished by that - and for giving the information so close to the release. There is nothing wrong about being pissed if the product you've ordered is delayed, even if it means you will get a better product in the end.
Personally I don't mind - I can wait for one or two months longer as I had no plans of taking time off to play WoW. But I perfectly understand those that did and are pissed because of it. Refunding it if you enjoy the game and want to play it is just childish rage though, but being upset is normal.