1. DUCK! If you’re going to jump out and take point then duck, I can’t stress this enough. (This applies to both gun and melee equipped players. This way you avoid friendly fire and you create a nice sweeping motion across the map.
2. INFORM people of your motives. If you’re going for an achievement, tell the group, that way they can either tell you 1) we aren't doing achievements or 2) agree to help.
3. COMMUNICATE! If someone is telling you that you are too far behind, too far ahead, or just in general not being a good team player, do not make excuses. Take it as constructive criticism and adjust, no one likes to play with someone where the constant discussion is:

-Why did you go up there without help?
-Because I wanted to see what was up there.
-And now you’re dead.

Some random notes:

• Feel free to use melee weapons, but to be honest when more than two people have melee weapons it’s annoying.
• Use pistols or your melee weapon until you really need to use your more powerful weapon (ie: when there are any special infected or uncommon infected. This conserves ammo).
  
• Make use of explosive and such; don’t always wait for someone to tell you to use it. Intuition is a key to survival.
• REMEMBER: Left 4 Dead 1 & 2 are not Halo do not spam obscenities or immature remarks (or noises) over the mic, some people may find it’s funny, but I can guarantee in a real Zombie Apocalypse I bet they would kill you and just claim you were infected.
• STRESSED NOTE: DIC! Duck, Inform, Communicate. When it comes down to it if you’re not communicating your team is probably going to fuck up badly. Take all criticism, it’s all probably true. Play the game and survive.