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