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