I'm writing this blog as a reaction to Jim's post earlier today. While I agree with Jim that the live community has been plagued for some time by a certain caste of gamers, I feel he undermined his post by the use of profanities, name calling, and sweeping generalizations. So here I am with a much more positive reaction to people who don't seem to know how to play L4D properly.
You'd think these would be common sense, but as I've discovered playing random games while waiting for my gf and friend to get online they aren't.
(If I missed anything, which I'm sure I did, please suggest it and I'll update the list as I feel necessary and of course give you credit where it is due)

1. Duck!
If you're going to be the first one through every door way then have the common sense to crouch. This way your teammates can actually shoot without having to worry about taking you out along with whatever might pop out. It's as easy as the press of a button and you get the bonus of better accuracy and not having to worry about accidental friendly fire.

2.Stick with the group
I realize that some people like being "in the action" but when you're running ahead of your teammates your putting both you and them at an unnecessary risk that will more than likely result in your team having to save you later. If you must go ahead then take point, but remember rule 1 and never get any further ahead of your team than they can accurately shoot at.

3. You have a melee, use it!
In my time with L4D, I've found that most friendly fire is either the result of people not paying attention to my 1st rule or senselessly shooting when melees are just as effective. Sure you can take the infected down faster by shooting them, but what good does it do when your friend takes more damage from your shots than the infected would have dealt had he taken care of them himself?

4.You don't have to kill special infected to get them off
To an extent this isn't much different than rule 3, but I've noticed most people prefer to kill the special infected that may have a friend unable to move instead of knocking them off. Sure sometimes it's quicker to go ahead and shoot that smoker, but what about if you can't see it? I consider L4D a survival action game (though others might say survival-horror, I really didn't find it that scary) with an emphasis on survival, every bit of health is precious and by trying to go after that smoker while leaving your friend tied up they're loosing it. Just knock the tongue off him and find the zombified Pigpen before his attack recharges.

5.Health is the team's
Look at that, there's four health packs, one for each of you, once you use yours better hope you find another or you're SOL right? Wrong! Don't be stingy with your health if you're sitting at 80% while your friend is bleeding out do the right thing and heal him, if he's worth anything as a teammate he'd do the same for you.

6.Don't use a health pack when pills will do
For me this ties right back into the survival aspect of the game. Both pills and health packs are a commodity, but health packs are in shorter supply. If you're hurt, but not bleeding out use those pills instead of wasting a health pack, you'll be glad you did when you get the message telling you your last knock down means death.

7.Help a downed teammate
This seems like it should be common sense, but I've seen numerous occasions where someone would get incapacitated and instead of taking the five seconds to pick them up the team just left them to die. This is a team game if that's not obvious and you need all four people to be at your peak, so it's in your best interest to make sure everyone makes it through alive.

8.Don't startle the witch!
Another common sense rule that I see a number of people disregard. When the team turns off their flashlights and starts sneaking around, checking every corner for the emotional wreck that is the witch it does them no good for you to do your best Steve Irwin impression and go agitate her.

9.The horde warnings are there for a reason.
I know how tempting it is to do something once someone tells you not to, but you're not winning any friends when you set off the car alarm and alert the horde; plus, the "I didn't know you could set if off by jumping on it," excuse only works once. Also, if your team is gearing up for a big standoff with the horde, don't be a wiseguy and activate whatever it is while everyone has their pants down.

10.There's no "I" in "team"
Yes it's a cliche, I'm fully aware of that, but this is a great governing principle when trying to fight off the infected. I think every one of my other rules can be summed up with this one, when you're doing something think of the other 3 too. Running through a campaign of L4D should bring everyone together so when you get to the final credits everyone can laugh at the statistics and not see, "In memory of ___"