You need a mic to play L4D2. If somebody on your team doesn't have a mic, vote kick them. Nothing is more annoying than having a silent teammate dying from a special infected because they didn't say anything.
Always know where your teammates are and what they're doing. Stay within a few steps of them at all times. Never wander off and yell at people who do. Don't go shopping for items. Countless times I've seen someone lose 50 health because they're trying to go shop for pills which will only give them 50 temporary health anyway.
If you get boomed, don't stand still and wait for it to wear off. Push forward through the zombies so that you all don't end up common locked in spit.
Nudge special infected when they attack. You can dead stop hunters, boomers, spitters, and even chargers to cancel their special attacks with a nudge.
Learn how to crown witches: just get super close and put the shotgun barrel down into her body and unload into their hips. If you don't have a shotty, you can just aggro them from far away and run and let your teammates kill her. You can also burn her.
Don't get close to the tank. Especially when there's hittable cars and stuff around.
To kill a jockey, backpedal away from it while shooting. If it gets close nudge it repeatedly or nudge it then shoot it.
To kill a hunter, shoot it while strafing to avoid getting pounced. If it's flying straight at you and you can't dodge it, nudge it as it's flying into you.
To kill a boomer, nudge it to get some distance and backpedal, then once it's clear from you and your teammates shoot it. Never shoot a boomer that will explode on anyone.
Spitters are easy to kill. Just shoot it. Sometimes it it's helpful to intentionally get hit by its spit to prevent the spit from hitting your teammates who are more vulnerable to the spit than you (common locked, incap'd, etc.).
To kill a tank, light it on fire, then keep your distance and shoot it. A good player can dance around a tank and not get hit a single time. If you're not that good yet, then just keep your distance.
Sometimes it's safe to walk around with low health. In those cases, hang onto your med pack instead of using it. If there's a tank, event, or risky ledges, then you should heal up to walk fast.
General Infected:
Always attack together. If you attack alone, you will die quickly and not do much damage.
Don't spawn early. You should spawn at the latest possible moment so that they don't know you're there.
Work with your other infected so set up charge spits, smoke charges, jockey glitches, jockey charges, or hunt spits.
If your teammate has landed an attack on a survivor, it's your job to protect that teammate to make their attack last as long as possible. The highest priority attack is the one on the survivor trying to free a teammate from a special attack.
Charger:
It's usually about the death charge. Know your death charge spots and learn how to pull them off (windows, ledges, into fire, etc.). Some death charges require multiple survivors to be bunched together, others require help from a smoker, and some you can pull off on your own. Other uses for the Charger are for a "charge spit" which is a combined charger and spitter combo, a group charge when the survivors are all bunched together, or charging a survivor who walked off on his own. When the survivors are injured and/or blind, sometimes it is preferred to just punch them to incapacitate multiple survivors. You should also resort to punching if you miss a charge. Sometimes you can actually get more damage from punches than you would have got with the charge. They do 10 each normally, or 20 each if you punch someone who is jockied. This is known as the "jockey glitch" and some people will bitch and moan if you do it.
Hunter:
The hunter does a ton of damage, but can't take much damage himself. Therefore you should never attack a survivor who is near his teammates unless the teammates are distracted, blind, or pinned by another special infected. The hunter is the quickest way to finish off a downed survivor. The hunter has no recharge time. All of the other special infected need to wait for their special attack to recharge. You can get bonus damage by jumping onto a survivor from a high position.
Jockey:
The jockey doesn't do much damage, but it can move a survivor. Your goal should always be to separate the survivor from his party before they can kill you. Duck behind the closest wall for cover once you've landed an attack. You can also walk them off of ledges, into fire, out windows, in front of charger attacks, etc. 9 times out of 10 the jockey is the worst infected to have. However, there is that one time when you will save the day with a well timed jockey attack.
Spitter:
Spit attacks are only effective if the survivors are stuck in the spit. They can get stuck by having a common infected horde on them, by being incapped, or by being pinned by another special infected. Once in a while it might be helpful to spit even if they won't be stuck in it. If you spit ahead of the survivors in a narrow place, they will have to wait for the spit to clear before moving on. This gives your teammates time to spawn again. The spitter does the most damage of all, but only if you can get the survivors stuck in it.
Tank:
The most useful tank is the long lasting one who keeps his distance and pesters them with rocks. If you must rush in on the survivors, you need to use the cars, garbage cans or fork lifts to instantly incap them. Trying to punch the survivors to death is usually a futile effort, unless they're already hurt, stuck in a zombie horde, or dealing with your teammates' attacks. You need to dodge the fire attacks or you'll die pretty quickly.
