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