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