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