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