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