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