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