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