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