I really love L4D. But with that being said, there are some extremely annoying parts of it.
1) Inability to stop loading a game, or stop reconnecting. The game shouldn't be "hijacked" in those cases. I should still be able to get to the menu.
2) Make Versus "fair". Yeah, you're never going to have even teams, but the reality is, what can work fine in Campaign mode really screws you up in Versus. I've had the "enemy" team get a tank right out of the door, while we didn't get a tank at all. The enemy team gets weapon upgrades at the first table, ours are at the last. The enemy team gets -two- tanks, we only get one. The enemy team gets an extra tank on the last level, we don't. In Campaign mode, that's just the luck of the draw, but in Versus, you are competing against the other team. Allowing them to get an extra tank, early weapons, etc, makes for extremely unbalanced games.
3) Find decent servers. The number of times I've had games end because of crappy servers is much higher than it should be. If you're going to have it search for servers, have it find decent ones.
4) Stop having me try to join "full" servers. Or, rather, if I select "join a game in progress", don't simply tell me the game is full, then kick me back to the main menu. I wanted to join a game - keep trying to join one until you manage to do it.
5) Don't let the enemy get into the exit vehicles. At the end of ... crap, can't remember the map now, but the Versus mode that isn't Mercy, the "final" spawned tank, that shows up when the exit vehicle gets there almost always manages to beat the survivors to the vehicle, even if they are right in the vicinity.
6) Don't allow friendly fire before the safe room door is opened. Too many times have I had someone incapacitate someone right at the start of a level - either because they are about to leave, because they were spamming the "shoot" button at the beginning of the round, or because they are idiots. The infected can't spawn until the survivors leave the safe room anyway.
7) Make some way to adjust the difficulty level with Versus. Sure, a lot of people might like it on Normal, but I, and I know I'm not alone here, find it too easy. Allowing the difficulty level to be set (before the game is launched) should be an option.
Those are the main ones I remember, there are probably a few I'm forgetting.