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