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