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