I've not had any major problems with L4d just a few minor ones I've observed are:
1. The whole team spawning as dead in campaign mode, this has happened twice in No Mercy after entering the safe house in the hospital... very strange, red tint to the screen as well.
2. On the final campaign (Bloody harvest is it called? I forget), safe house door wouldn't open after loading the next section.
3. Occasional crash to desktop with no error messages, probably happened 2 or 3 times since I got it.
4. Occasional crash when shutting down the game with "programme is not responding" message.
The lobby system seems fine to me, I've had no problems with it at all. sometimes I can't find a server to start a game (might download the dedicated server and host myself sometimes).
As for negative feedback, I'd second that: I've had people run off on their own being abusive when the rest of the team couldn't get to them on time. People turning on the rest of the team for no apparent reason and we couldn't take them out because the AI kept reviving them! Overall though I've played with some very nice people including someone from hexus
