Here is the full list of all 73 Left 4 Dead achievements. It takes around 60-80 hours to unlock all of the achievements on Windows.
Kill 53,595 Infected.
Kill 5,359 Infected in the Crash Course campaign.
Kill 1000 Infected with a mounted machine gun.
Catch a rare strain of infection, then pass it on to someone else.
As a Special Infected, incap someone who is trying to sacrifice themselves.
Have all 3 generators running at once in "The Sacrifice" finale.
Kill a Special Infected with an exploding barrel.
Save another Survivor from a Special Infected while on the ground.
Survive all campaigns on Expert.
Kill a Witch with a single headshot.
No Survivors take damage after contacting the rescue vehicle.
Survive a campaign with no Survivors taking damage from Special Infected.
As an Infected, incap a Survivor who has entered and left a safe room.
Dole out 5000 total Survivor damage as a Special Infected.
As A Tank hit 20 Survivors with a car in the Crash Course campaign.
Win a Versus campaign of Crash Course.
Stun a Tank with an explosion in the Crash Course campaign.
Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
Kill all four Survivors on one life as a Tank.
Your team incapacitates three Survivors within five seconds.
Cover four Survivors with Boomer bile at once.
Smoker pull a bile-covered Survivor until you hold him during Crash Course.
Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Constrict two Survivors on one life as a Smoker.
Pounce two different Survivors on one life as a Hunter.
Drag a Survivor 100 feet with your tongue as a Smoker.
Earn a Gold medal on every official Survival level.
Earn at least a Silver medal on every official Survival level.
Earn at least a Bronze medal on every official Survival level.
Revive 50 incapacitated Survivors.
Leave a safe room to save an incapped teammate and bring them back safely.
Shove a Hunter off of a pinned and helpless Survivor.
Complete one round of Survival on The Last Stand.
Single-handedly kill a Tank.
Headshot a Hunter while he's leaping.
Earn a Gold medal in Survival mode on any official level.
Earn a Silver medal in Survival mode on any official level.
Earn a Bronze medal in Survival mode on any official level.
Give pain pills to 10 Survivors.
Survive any campaign on Expert.
Set 101 Infected on fire.
Play an entire campaign with no Survivors taking friendly fire damage.
Kill a Witch without any Survivor taking damage from her.
Restart the generator within 30 seconds of it shutting off.
Sneak past all Witches in a campaign without disturbing one.
Finish a campaign without ever using a first aid kit on yourself.
All Survivors complete a campaign without being vomited on.
Survive the Crash Course Campaign.
Complete "The Sacrifice".
Have Bill sacrifice himself for the team.
Shove a Boomer and then kill him without him splashing on anyone.
Light a Tank with a Molotov.
Kill 10 Smokers as they are pulling helpless Survivors.
Survive the Blood Harvest campaign.
Heal 25 Survivors with a first aid kit.
Kill an Infected with a single blow from behind.
Survive an entire campaign using only pistols.
Survive the Dead Air campaign.
Protect any Survivor from an attacking Infected 50 times.
Survive a campaign without being incapacitated.
Survive the Death Toll campaign.
Beat a campaign with all 4 Survivors.
Heal a fellow Survivor when your own health is below 10.
Punch a Hunter as he is pouncing.
Rescue a Survivor from a Smoker's tongue before he takes damage.
You or another Survivor take no damage after being vomited on by a Boomer.
Rescue a Survivor trapped in a closet.
Kill a Smoker who has grabbed you with his tongue.
Kill a Tank without it dealing any damage to a Survivor.
Blow up 20 Infected in a single explosion.
Light a Witch with a Molotov.
Make 100 headshot kills.
Survive the No Mercy campaign.