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