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