Mismatches. Matches that spend at least 1:30 minutes (or so) as anything other than full 5v5 will automatically not be counted to determine MMR or leaderboard rank.
Surrender. Matches where one team is up by around 250 points or more are neither fun to play nor watch. If at least 3 members of a team agree, they can forfeit the game early without accruing Dishonor.
Map preferences. You can select one or two maps that you would never like to play on, voluntarily accepting longer queue times if you wish, and the queue system will try its best never to place you in a match on those maps.