Yellow-bellies beware! Valve are cracking down on the cowards who flee games prematurely, leaving their team-mates in the lurch just to avoid an X in their ledger. The changes affect Mann vs Machine mode only, at the moment, and have been designed so as not to penalise people who get booted by the odd connection issue.
The Mann vs. Machine FAQ breaks the changes down. You are only ever punished for abandoning a game. This is different from leaving a game. Abandoning happens when:
You leave a game while playing a Mann Up game on an official server; and
You have not played through a single wave to completion or failure; and
There are 5 or more players currently connected to the server (including you).
Otherwise, you are cool to quit. In fact, even if you do satisfy the abandonment criteria, you will not be punished - at first. The punishments only kick in for repeat offences, though it's not specified exactly how many or often. Luckily, if you leave the game by pressing the disconnect button, you will be informed if you're at risk of punishment.
Said punishment means being placed in low-priority matchmaking pools, making it longer to get into a game.
If you just crash out, or your connection drops, your spot will be reserved for three minutes, allowing you to rejoin without incurring any penalty. Getting kicked for idling is considered the same as quitting intentionally, so you can be penalised. However, being vote-kicked does not count, and you won't be penalised for leaving. Though you may still be an asshole, which is a kind of punishment in itself.
So, quitters, cowards and wheyfaced weaklings - you have been warned. In the words of Saxton Hale, the not-actually-real-though-I-wish-he-was President and CEO of Mann Co:
"I gave you mercs one simple thing to do: Defend Mann Co. from an unstoppable robot horde. Now Bidwell's telling me some of you are abandoning the fight in the middle of attacks. Firstly: What am I even paying you in found money that falls out of robots for?
Secondly — actually, put this first, it's more important — I am coming for you."