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