Communication, or lack thereof. There are so many people with great ideas, or good reasons to not like an idea, that were effectively ignored. Game changes were implemented that nobody wanted, and game changes everyone desperately wants were never even talked about by any of the red-names.
Performance. If you had average or worse computers/internet, little hiccups would occur all the time. If it were a coop game that's fine, but when you're about to make a game-deciding kill and your client