
Left 4 Dead continues to be a favorite game of mine, and the more I play it, the more I've learned a little 'Deadiquette' to make playing with fellow zombie-shooters a better experience for everyone.
1. Duck, dammit.
Friendly fire is a fact of the game -- especially if you're swarmed by zombies and some team mates are trying to help you out. But if you're going to constantly take point and rush out ahead of the group, crouch! (And if you don't, don't complain if you eat a few bullets in the back from team mates.)
2. Inform team mates in advance about any achievements you're shooting for.
Look, we all like to get the 'Akimbo Assassin' achievement (finish a campaign using only a pistol/twin pistols), but let everyone know in the game lobby so when a Tank starts barreling into the group they'll know you won't be much help as far as firepower goes.
Another achievement that you should warn your group you're going for is 'Burn the Witch' (use a Molotov to ignite the Witch). Sometimes folks just want to pass by the witch without pissing her off -- especially on Advanced or Expert levels -- so for the love of all that's holy, warn the other players before you go igniting her and she tears into you. She's a bad mamma.
3. Communicate.
There are a lot of good L4D players out there -- but apparently they've gotten so good they say nary a word for the entire campaign. Don't assume everyone automatically knows the strategy for a particular location -- especially for areas where there is more than strategy. And don't forget to stick together as a group -- I've seen some players who clearly know they're doing all to well, and just shoot out ahead of the group without a word. (Good luck, Rambo.)
In addition -- and this is just my own opinion/preference -- I generally enjoy the game more if my human teammates are more interesting than AI players. A little wisecrack every once in a while or strategy suggestion is nice to hear. I like to be reminded that I'm not playing with bots.
Read a list of Left 4 Dead Achievements at Left4dead411