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