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