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