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