The Definitive L4D Newbie Guide

So I've decided to compile a list of all the things a new player would need to know if they don't want to make it look like they just finished downloading the game on steam.  I constantly see players who have been playing the game a while who still don't know some of the basics so hopefully this will help everyone.  I'll try to break it down into sections to group related things.

General Tips:

Survivors - NEVER split up

As Infected - ALWAYS attack together

Hunters:

Damage Pouncing - Hunters can do additional damage to survivors based on the distance traveled from their last leap.  Arc does not factor into this, just simply a straight line distance from point a to point b.  This damage is capped out at 25

Air Control - Hunters have a decent amount of leeway to adjust their direction in the air.  You can you strafe and mouse look to "curve" your jumps.  In addition to this if you hold crouch your air control becomes much more limited, however if you miss your pounce and land on the ground you can re-pounce almost immediately instead of having to re-crouch after landing.

Wall Jumping - Hunters have the ability to jump off surfaces to help them move around the map more swiftly, as well as set up for high damage pounces that wouldn't normally be possible.  You don't need to hold crouch to do this, but your mouse must be angled at least about 10 degrees in the opposite direction of the object you wish you re-pounce off of.  There are more advanced tactics of wall jumping

(best example of whats possible - http://www.youtube.com/watch?v=W3O9p_Ru-BQ )

Slash damage - Hunters secondary melee attack does 6 damage per swipe, and in addition to this you can hit all targets within a small area in front of the hunter.  This means if survivors are packed in tight quarters it is better to run in and slash them then try to pounce them.  Shred damage on a pounced target is 5 damage every .55 seconds so unless you can get a couple ticks of shred damage in it is often better to slash survivors especially if they are grouped up.

Fire - You can do increased damage when on a pounced survivor if you are lit on fire, but only from a molotov or gas can, static environmental fires do not cause additional damage anymore.

Pounce Knockback - When you pounce a survivor it causes an area effect knockback that will render the survivors unable to do anything for 1-2 seconds if they are close to the survivor you pounced.

Smokers:

Tongue Breaking - There are a handful of things that cause the tongue to break.  If a survivor breaks line of sight of your tongue while its shooting, your tongue will break.  If a survivor gets meleed a second after you grab him, your tongue will break.  If a survivor shoots your tongue, it will break.  Also your tongue cannot pass through other special infected, but can now pass through common infected.

Pulling survivors - Survivors will get caught on objects or common infected when you pull them, once they get caught on something your damage will increase from 3 damage every 1.1s to 5 damage every 1.1s.  Also you can drag survivors off certain ledges or ladders and cause massive amounts of fall damage, these are the spots where you can really shine (Ladders are your best friend).

Ledge Incapping - You can instantly incap, and sometimes instantly kill any survivors by pulling them off any ledge that would cause them to fall to their death if they jump off.

Boomers:

Puke Delay - When you spawn your vomit has about a 1-1.5 second delay before you can actually vomit.  Make sure you have adequate time after spawning for this delay to go away before trying to puke on survivors.  Also when you do puke there is a slight delay before it actually lands where survivors can melee or shoot you and even though it looked like you vomited on them it will not land.

Resummoning Hordes Suiciding - You cannot resummon a new horde by re-biling the survivors due to suiciding on them until the initial vomit has just about worn off.  I believe the exact time is about 20 seconds, possibly 25 before you can re-horde them after the initial vomit.

Vomit Arc - The furthest point of your vomit will always be the initial spray, as the distance of the vomit rapidly reduces during the rest of the vomit.

(more on this - http://www.youtube.com/watch?v=y8ovlRunByg )

Explosion Knock back - When the boomer explodes he causes a knock back similar to the hunters pounce, however this knock back will also knock back tanks if they are close to the boomer

Tanks:

Fire - Fire IS NOT YOUR FRIEND, stay away at all costs.  When lit on fire not only do you take damage over time, and in addition to this you are slower than when un-lit.  The damage over time is about 100 damage every 2 seconds.

Water - You can use water to put yourself out if you get lit on fire.

Range - You take greatly reduced damage when you are far away from the survivors, try to keep range and throw rocks until you have the upper hand when the survivors become slowed by common infected.

Tank Punch - Your tank punch is able to hit multiple survivors, punish them if they get grouped up.

Slow - Whenever you take damage your movement speed is slowed, it is unwise to try and chase survivors unless you have help from the team if they aren't in red health.

Rocks - Rocks can be shot out of the air by most of the survivors weapons.

Cornering - A great tactic for a tank to use is to try and corner a survivor.  This is because your hit box will block them from running away from you and you can just punch them again and again until they are downed then chase the rest of the survivors.

Hittable Objects - Most hittable objects will instantly incap a survivor, however some of the smaller ones will simply cause damage equal to thrown objects (pillars and the small pallet pullers are in this category).  Hay bales, tree logs, forklifts, cars, luggage containers, and dumpsters are all examples of things that will incap survivors.

Propane Tanks + Oxygen Tanks - Both of these objects cause an explosion that like the boomers explosion will stagger the tank for about 2-3 seconds.  Also on levels like no mercy 4 they have the ability to stagger the tank off the level so be wary of them.

Witches:

Shotguns - Shotguns are the only weapon that can kill a witch in one shot as they do x5 damage at point blank range.

Aggro - You can send the witch after the survivors by meleeing her as soon as she starts getting even just a little bit angry when the survivors approach her.  However the survivors must still be in aggro range when you melee her.

Incap - The witch will incap any survivor she hits, then after a few seconds will do massive damage to them killing them in seconds.

Health:

Health Bonus - Your teams health bonus is the individual health bonus of each player added together.  Everyones individual health bonus can be calculated by Health / 2 + Temp Health / 4.  When you close the safe room door any items you have are automatically "used" you.  However if someone has very low health with no medpack, and you have high health with a medpack you will get a higher score by healing them, just make sure they take damage to update the bonus before closing the safe room door.

Health Kits - Health kits will heal you for 80% of your missing health, this does not take into account temporary health.

Temporary Health - Temporary health decays over time, and in addition to this if you still have normal health remaining, you will lose your normal health first when taking damage before temp health.

Survivors:

Melee Fatigue - After a certain number of melees (3 I think) you will experience melee fatigue which greatly increases the cooldown of your melee making it unreliable to "spam" in an effort to clear common infected or melee hunters.

Shotgun + Melee Combo - You are able to melee almost immediately after firing, this becomes especially useful when using the shotgun (T1 specifically).  This allways you to fire at pouncing hunters and melee at almost the same time incase you miss greatly increasing the chances of denying their pounce.

Meleeing Special Infected - With the exception of the tank and witch, melee will disrupt the special attacks of all the other Special Infected.  On hunters, it will knock them off the grounded target stunning them for approximately 1-2 seconds and also "blinding them".  Smokers will have their tongue released and also suffer the "stun" and "blind" effects that hunters do.  Boomers on the other hand can sometimes have their vomit interrupted if they are meleed quick enough after they start to vomit, this also triggers the vomit cooldown rendering them useless save for their explosion vomit.

Infected Spawning:

Restrictions - There are a couple of spawning restrictions in regards to special infected.

1) Distance - You cannot spawn too close to the survivors, however the range is fairly small

2) Line of Site - You cannot spawn in the line of sight of survivors, however this does not include the width of your character model so thin poles and trees often prove for adequate places to spawn.  Height does however matter, and sometimes you will need to crouch jump to spawn in areas where there might be line of sight under an object (cars).