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 Witch with a Molotov.
Light a Tank with a Molotov.
Kill an Infected with a single blow from behind.
All Survivors complete a campaign without being vomited on.
Make 100 headshot kills.
Headshot a Hunter while he's leaping.
Survive the No Mercy campaign.
Survive the Death Toll campaign.
Survive the Dead Air campaign.
Survive the Blood Harvest campaign.
Constrict two Survivors on one life as a Smoker.
Pounce two different Survivors on one life as a Hunter.
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.
Punch a Hunter as he is pouncing.
Rescue a Survivor from a Smoker's tongue before he takes damage.
Kill a Witch with a single headshot.
You or another Survivor take no damage after being vomited on by a Boomer.
Protect any Survivor from an attacking Infected 50 times.
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.
Rescue a Survivor trapped in a closet.
Shove a Hunter off of a pinned and helpless Survivor.
Kill a Smoker who has grabbed you with his tongue.
Kill 10 Smokers as they are pulling helpless Survivors.
Set 101 Infected on fire.
Single-handedly kill a Tank.
Kill a Tank without it dealing any damage to a Survivor.
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.
Blow up 20 Infected in a single explosion.
Heal a fellow Survivor when your own health is below 10.
Save another Survivor from a Special Infected while on the ground.
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.
Drag a Survivor 100 feet with your tongue as a Smoker.
Complete "The Sacrifice".
Have Bill sacrifice himself for the team.
Sneak past all Witches in a campaign without disturbing one.
Win a Versus campaign of Crash Course.
No Survivors take damage after contacting the rescue vehicle.
Dole out 5000 total Survivor damage as a Special Infected.
Kill 1000 Infected with a mounted machine gun.
Survive all campaigns on Expert.
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.
Kill 53,595 Infected.
Play an entire campaign with no Survivors taking friendly fire damage.
Survive a campaign with no Survivors taking damage from Special Infected.
Cover four Survivors with Boomer bile at once.
Kill all four Survivors on one life as a Tank.
Earn a Gold medal on every official Survival level.
Kill 5,359 Infected in the Crash Course campaign.
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.
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.