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