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.
Survive all campaigns on Expert.
Earn a Gold medal on every official Survival level.
Earn at least a Silver medal on every official Survival level.
Kill 53,595 Infected.
Earn at least a Bronze medal on every official Survival level.
Complete one round of Survival on The Last Stand.
As a Special Infected, incap someone who is trying to sacrifice themselves.
Catch a rare strain of infection, then pass it on to someone else.
As A Tank hit 20 Survivors with a car in the Crash Course campaign.
Dole out 5000 total Survivor damage as a Special Infected.
Your team incapacitates three Survivors within five seconds.
Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
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 all four Survivors on one life as a Tank.
Constrict two Survivors on one life as a Smoker.
Pounce two different Survivors on one life as a Hunter.
As an Infected, incap a Survivor who has entered and left a safe room.
Cover four Survivors with Boomer bile at once.
Drag a Survivor 100 feet with your tongue as a Smoker.
Earn a Gold medal in Survival mode on any official level.
Earn a Silver medal in Survival mode on any official level.
Earn a Bronze medal in Survival mode on any official level.
Kill 5,359 Infected in the Crash Course campaign.
Kill 1000 Infected with a mounted machine gun.
Stun a Tank with an explosion in the Crash Course campaign.
Heal a fellow Survivor when your own health is below 10.
No Survivors take damage after contacting the rescue vehicle.
Survive an entire campaign using only pistols.
Save another Survivor from a Special Infected while on the ground.
Survive any campaign on Expert.
Play an entire campaign with no Survivors taking friendly fire damage.
Complete "The Sacrifice".
Have Bill sacrifice himself for the team.
Sneak past all Witches in a campaign without disturbing one.
Finish a campaign without ever using a first aid kit on yourself.
Survive a campaign with no Survivors taking damage from Special Infected.
All Survivors complete a campaign without being vomited on.
Win a Versus campaign of Crash Course.
Restart the generator within 30 seconds of it shutting off.
Revive 50 incapacitated Survivors.
Kill an Infected with a single blow from behind.
Single-handedly kill a Tank.
Heal 25 Survivors with a first aid kit.
Punch a Hunter as he is pouncing.
Give pain pills to 10 Survivors.
Kill a Special Infected with an exploding barrel.
Light a Tank with a Molotov.
Have all 3 generators running at once in "The Sacrifice" finale.
Set 101 Infected on fire.
Survive the Crash Course Campaign.
Survive the Blood Harvest campaign.
Headshot a Hunter while he's leaping.
Light a Witch with a Molotov.
Leave a safe room to save an incapped teammate and bring them back safely.
Survive the Dead Air campaign.
Protect any Survivor from an attacking Infected 50 times.
Survive the Death Toll campaign.
Beat a campaign with all 4 Survivors.
Shove a Hunter off of a pinned and helpless Survivor.
Kill a Tank without it dealing any damage to a Survivor.
Kill 10 Smokers as they are pulling helpless Survivors.
Survive a campaign without being incapacitated.
Survive the No Mercy campaign.
You or another Survivor take no damage after being vomited on by a Boomer.
Make 100 headshot kills.
Shove a Boomer and then kill him without him splashing on anyone.
Blow up 20 Infected in a single explosion.
Kill a Witch with a single headshot.
Kill a Witch without any Survivor taking damage from her.
Kill a Smoker who has grabbed you with his tongue.
Rescue a Survivor from a Smoker's tongue before he takes damage.
Rescue a Survivor trapped in a closet.