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.
Kill 10 Smokers as they are pulling helpless Survivors.
Survive the No Mercy campaign.
Protect any Survivor from an attacking Infected 50 times.
Rescue a Survivor trapped in a closet.
You or another Survivor take no damage after being vomited on by a Boomer.
Survive the Death Toll campaign.
Survive a campaign without being incapacitated.
Beat a campaign with all 4 Survivors.
Survive the Crash Course Campaign.
Restart the generator within 30 seconds of it shutting off.
Set 101 Infected on fire.
Make 100 headshot kills.
Shove a Boomer and then kill him without him splashing on anyone.
Kill a Tank without it dealing any damage to a Survivor.
Blow up 20 Infected in a single explosion.
Kill an Infected with a single blow from behind.
Rescue a Survivor from a Smoker's tongue before he takes damage.
Kill a Witch without any Survivor taking damage from her.
Kill a Smoker who has grabbed you with his tongue.
Punch a Hunter as he is pouncing.
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.
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.
Heal a fellow Survivor when your own health is below 10.
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.
Light a Tank with a Molotov.
All Survivors complete a campaign without being vomited on.
Headshot a Hunter while he's leaping.
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.
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.