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