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