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