Blizzard has detailed the penalty for frequent match-quitters in Overwatch.
Having someone leave in the middle of a match is frustrating. Getting dumped into a match that has already started - or worse, one that ends 20 seconds after you jump in - is also frustrating. Both of these are the result of match-quitters, and Blizzard is taking measures, via penalties, to try to combat how frequently this is happening in Overwatch.
On the Battle.net forums, Blizzard recently detailed the penalty for habitually quitting matches. There is an algorithm in place that evaluates your previous 20 matches, and how many of them were complete. Without saying what the magic number is, Blizzard says if the ratio of not completed to completed matches doesn't hit that number, players will receive a warning in the lower left-hand corner of the screen. If you keep quitting anyway, experience will be docked 75% in future matches, until you hit an appropriate 20-match ratio. Disconnecting, or being removed due to inactivity, counts just like if you left intentionally.
Blizzard also clarified some more specifics: If you leave a game while you're still on the screen to assemble your team (prior to entering the spawn room), it will not count as a game played, and therefore will not count as leaving. Leaving after you see the Victory/Defeat screen will count as a game played, but you will not count as a leaver (so you don't have to sit through the POTG or anything beyond the Victory/Defeat screen). You are only considered a leaver after the team assembly and prior to the end of the match.
"Leaving a couple games in a row won't cause you to be penalized in any way," an edit to the post assured players. "We understand that there are circumstances, either in or outside of your control, that may necessitate you needing to step away from a game before it ends. It's only if you repeatedly leave games in succession, to the point where it could be considered a habitual behavior, that you will be penalized."