Now there is a difference between working as a group and just plain slow moving group (I got this too with PUGs) who don't move very fast and WANT to kill everything (you can't really since director just respawn more) the object of the game is to get to the next safe house and win the scenario as a team
Pacing a team is difficult. Trying to kill everything is not only pointless, but counter-productive since it tends to piss off the AI Director. (BTW, if you're in the safe room waiting for someone to come back form AFK, don't shoot the zombies through the door. Racking up lots of kills will only cause a boatload of trouble when you finally go through the door.) However, being impatient and trying to rush a little extra distance before a horde hits is also a bad choice, if you end up in the open when you could have a wall at your back or a door for a choke-point. Plus, I think it pays to explore a bit if the
team needs health packs, pills or grenades. I've seen too many players with full inventory, and green health, rushing ahead while their teammates are orange and red with-out any items. The game shows you teammate's inventories for a reason, and it's good to keep an eye on what everyone has.
My other pet peeves:
1. Leaving behind defibs and special ammo when they don't have a health kit.
2. Not using pills when seriously low on health.
3. Not pointing out items to other players.
4. Dancing around like they're playing Quake, so I can't provide cover fire without hitting them.
5. Standing up on the front line. "Down in front!"
What I've been guilty of:
1. Not having a mic.
2. Stepping into the line of fire.
3. Itchy trigger finger and shooting survivors because I mistook them for a zombie.
4. Being totally oblivious to a pounced/smoked/etc teammate.
5. Dancing around like I'm playing Quake, especially with a melee weapon.
6. Spending too much time exploring, even when the team is well equipped.
7. Clinging too tightly to a gas can when I should have dropped it and started firing.
8. Running back for items instead of sticking with the group.