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