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.
Survive all campaigns on Expert.
Survive any campaign on Expert.
Earn a Gold medal on every official Survival level.
Earn at least a Silver medal on every official Survival level.
Earn at least a Bronze medal on every official Survival level.
Kill 53,595 Infected.
Survive a campaign with no Survivors taking damage from Special Infected.
Win a Versus campaign of Crash Course.
Your team wipes all Survivors after the escape vehicle has opened in Crash Course.
Catch a rare strain of infection, then pass it on to someone else.
Kill 5,359 Infected in the Crash Course campaign.
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 one round of Survival on The Last Stand.
Kill 1000 Infected with a mounted machine gun.
No Survivors take damage after contacting the rescue vehicle.
All Survivors complete a campaign without being vomited on.
As an Infected, incap a Survivor who has entered and left a safe room.
Smoker pull a bile-covered Survivor until you hold him during Crash Course.
As A Tank hit 20 Survivors with a car in the Crash Course campaign.
Dole out 5000 total Survivor damage as a Special Infected.
As a Special Infected, incap someone who is trying to sacrifice themselves.
Have all 3 generators running at once in "The Sacrifice" finale.
Your team incapacitates three Survivors within five seconds.
Revive 50 incapacitated Survivors.
Kill all four Survivors on one life as a Tank.
Pounce a Survivor for 25 points of damage in the Crash Course campaign.
Kill a Special Infected with an exploding barrel.
Single-handedly kill a Tank.
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.
Complete "The Sacrifice".
Have Bill sacrifice himself for the team.
Survive the Blood Harvest campaign.
Heal 25 Survivors with a first aid kit.
Survive the Dead Air campaign.
Survive the Death Toll campaign.
Kill a Witch with a single headshot.
Save another Survivor from a Special Infected while on the ground.
Play an entire campaign with no Survivors taking friendly fire damage.
Kill a Witch without any Survivor taking damage from her.
Light a Witch with a Molotov.
Restart the generator within 30 seconds of it shutting off.
Stun a Tank with an explosion in the Crash Course campaign.
Set 101 Infected on fire.
Survive the Crash Course Campaign.
Beat a campaign with all 4 Survivors.
Kill a Smoker who has grabbed you with his tongue.
Shove a Boomer and then kill him without him splashing on anyone.
Protect any Survivor from an attacking Infected 50 times.
Survive an entire campaign using only pistols.
Sneak past all Witches in a campaign without disturbing one.
Finish a campaign without ever using a first aid kit on yourself.
Kill a Tank without it dealing any damage to a Survivor.
Leave a safe room to save an incapped teammate and bring them back safely.
Headshot a Hunter while he's leaping.
Give pain pills to 10 Survivors.
Shove a Hunter off of a pinned and helpless Survivor.
Survive a campaign without being incapacitated.
Punch a Hunter as he is pouncing.
Kill an Infected with a single blow from behind.
Kill 10 Smokers as they are pulling helpless Survivors.
Heal a fellow Survivor when your own health is below 10.
Make 100 headshot kills.
Survive the No Mercy campaign.
Blow up 20 Infected in a single explosion.
You or another Survivor take no damage after being vomited on by a Boomer.
Rescue a Survivor trapped in a closet.
Rescue a Survivor from a Smoker's tongue before he takes damage.
Light a Tank with a Molotov.