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".
Have Bill sacrifice himself for the team.
Leave a safe room to save an incapped teammate and bring them back safely.
Kill 1000 Infected with a mounted machine gun.
Earn at least a Silver medal on every official Survival level.
Kill 5,359 Infected in the Crash Course campaign.
Survive any campaign on Expert.
Earn at least a Bronze medal on every official Survival level.
Survive an entire campaign using only pistols.
Stun a Tank with an explosion in the Crash Course campaign.
Single-handedly kill a Tank.
Dole out 5000 total Survivor damage as a Special Infected.
Smoker pull a bile-covered Survivor until you hold him during Crash Course.
Your team incapacitates three Survivors within five seconds.
Survive a campaign without being incapacitated.
Finish a campaign without ever using a first aid kit on yourself.
Survive the Crash Course Campaign.
Complete one round of Survival on The Last Stand.
Heal 25 Survivors with a first aid kit.
Revive 50 incapacitated Survivors.
Earn a Gold medal in Survival mode on any official level.
Give pain pills to 10 Survivors.
Headshot a Hunter while he's leaping.
Save another Survivor from a Special Infected while on the ground.
Constrict two Survivors on one life as a Smoker.
Shove a Boomer and then kill him without him splashing on anyone.
Protect any Survivor from an attacking Infected 50 times.
Survive the Blood Harvest campaign.
Kill 10 Smokers as they are pulling helpless Survivors.
Survive the No Mercy campaign.
Beat a campaign with all 4 Survivors.
Rescue a Survivor from a Smoker's tongue before he takes damage.
Catch a rare strain of infection, then pass it on to someone else.
Punch a Hunter as he is pouncing.
Kill a Witch with a single headshot.
No Survivors take damage after contacting the rescue vehicle.
You or another Survivor take no damage after being vomited on by a Boomer.
Rescue a Survivor trapped in a closet.
Shove a Hunter off of a pinned and helpless Survivor.
Kill a Smoker who has grabbed you with his tongue.
Set 101 Infected on fire.
Sneak past all Witches in a campaign without disturbing one.
Kill a Tank without it dealing any damage to a Survivor.
Play an entire campaign with no Survivors taking friendly fire damage.
Kill a Witch without any Survivor taking damage from her.
Blow up 20 Infected in a single explosion.
Kill 53,595 Infected.
Heal a fellow Survivor when your own health is below 10.
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.
All Survivors complete a campaign without being vomited on.
Make 100 headshot kills.
Survive the Death Toll campaign.
Survive the Dead Air campaign.
Cover four Survivors with Boomer bile at once.
Drag a Survivor 100 feet with your tongue as a Smoker.
Pounce two different Survivors on one life as a Hunter.
Kill all four Survivors on one life as a Tank.
As an Infected, incap a Survivor who has entered and left a safe room.
Earn a Bronze medal in Survival mode on any official level.
Earn a Silver medal in Survival mode on any official 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.
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.