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