I get what people are saying about team balance, but what really gets me salty are either:
1. Loading into a game with less than 60 seconds left in the match because somebody decided to quit out before seeing the Defeat screen (what does that even accomplish?)
2. Finishing a game and being sent to an empty Lobby (why can't I stay grouped with the same players and roll over to the next map like when Co-Op was first added?)
It used to be where you could start playing Co-Op and get into an enjoyable run of playing for an hour or more without interruption, now it's occasional gameplay punctuated by long periods of waiting for a new team.