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