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