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