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.

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.

 I do play with just my friends, but we've started to introduce some friends to pc gaming recently and it's been difficult. Plus this is just kind of a for-fun type of thing. I know most things won't improve but you know, i can dream and or make an Etiquette guide for shits and giggles.

These are some good rules, mrhankey.   I never thought about ducking while taking point.  Makes total sense.  I'm not a  very good player, but following these rules should help.

Don't waste a boomer!
Hate seeing people running towards the survivors from a distance.

These are for co-op campaign not versus, but so true.

Point man ducking is imperative to success.

This is why I hate online coop game, people always take this shit too seriously.

"@Diamond: I'm sorry?

I'm being difficult, because i wanted to see how you would react. however that may have been a mistake as i also don't want this to turn into a verbal pissing contest as to be honest, i don't care if you play online co-op games and you probably don't care if i do.

@Diamond: It's people like you that make playing cooperative games so frustrating... if you don't want to play seriously, then don't.  I'm sure there are plenty of other 12 year olds on the internet for you to dick around with.  This guide isn't trying to encourage people to be arrogant or no fun.... it's a few handy tips that are meant to inspire teamwork and coordination, two fundamentally important important aspects of cooperative gaming.

You are totally wrong about the melee weapons. A baseball bat is far and beyond better than having pistols, in all situations.

I prefer the chainsaw when i do use melee weapons