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