The biggest gripe I have is people leaving matches in team games, oftentimes right after the match starts. I've played several matches where someone leaves and causes an avalanche of disconnects, meaning you don't get anything for the "win" and the round is over before it even gets good.
Maybe have a League-style penalty system? Where if you leave a game you have to wait twenty or so minutes to find a game next time you automatch.
Wait 20 minutes only for a load screen bugsplat because the game wasn't restarted after the previous match.
A lot of people in 4v4s ragequit when there is an afker. And there's usually at least one in 4v4s. I often dub 4v4s to be AI v AFK mode.
Most of the outcomes of 4v4s are decided in the first 5 minutes, so people usually won't want to play a 50 minute lost cause after waiting 10 to 20 minutes for a game.