In the L4D2 trailer "and when we run out of bullets..." -cue chainsaw reving up-
"... they gonna wish we hadn't."
How about Coach's grandmother, during the trailer for the second game?
"Last time I saw my grandmother, she asked me, 'Was I still a praying man?' I told her, 'Yes ma'am.' 'Well,' she says, 'pray harder, 'cause it ain't workin.' "
Coach contacts the military, who ask him if he's encountered the infected. His reply: "Encountered?! Boy, I am covered in zombie blood and puke, and eyeballs, and twenty other parts I don't even recognize! We are as immune as shit!"
Have you ever killed a Hunter in such a way that he cartwheels back a few meters? Immensely satisfying.
Each campaign ends with a finale calling for a rescue vehicle to pick up the Survivors. As the vehicle arrives (or the fuel is pumped in the case of the airplane for Dead Air, or fueled by the Survivors in the case of Dead Center), the Survivors must defend themselves against waves upon waves of common and special infected and fight multiple Tanks (at least two). Once the rescue is ready, the Director throws everything possible at the Survivors to try to kill them before they can board the vehicle.
Not to mention that you sometimes face two Tanks almost simultaneously in "Swamp Fever"'s finale.
"Dark Carnival" takes the cake, though. To quote Ellis:
Ellis: So we gotta set up to rock, and then fight zombies. What next, dinosaurs flyin' down from the sky shootin' lasers outta their eyes?
In the community campaign "Death Aboard", you have to go to a house on top of a large hill, start up a machine, survive a horde and a couple of tanks, and then race to the bottom of the hill and escape in a hot air balloon.
The second half of "Hard Rain" takes place during a hurricane. It's more than just a set effect as might commonly be found in other video games. Areas your group previously passed through unhindered are now flooded knee-to-waist deep, and the rain and wind kick up at intervals. As a result, the game reduces how far you can see. Likewise the game not only sends the bass booming with the thunder and lightning, but also turns down the in-game voice, meaning that if you want to say anything, you have to shout over the din of the storm. Many players use an out-of-game chat client to coordinate their assaults, however.
The start of the finale in "Dead Air" features one of the most spectacular fake plane crashes ever.
There's one for any player who manages to take down a Tank single-handedly (Hint: Boomer bile and either Chainsaws or Auto Shotguns).
Pretty much anything a player does can be made awesome if it follows a well-timed melee attack to free himself from an incoming Smoker tongue. note To clarify, you're normally not able to attack with melee weapons while a Smoker is pulling you. Due to the delay between pressing the fire button and the attack actually occuring, however...
To clarify, Bill has had an irrational hatred/fear of the Tanks, pushing aside everything else until the Tank is dead. He charges headlong into battle with 3 of them armed with only a molotov and an assault rifle. As soon as he hits the generator, a Tank punches him in the head, sending him flying. And as he's dying, he doesn't cower. Instead, he lights a cigarette and tells the Tanks to Bring It.
In The Sacrifice campaign, you actually play through this, with any of the four survivors capable of doing it. Louis' final words are very badass, considering his normal fearful-yet-hopeful personality.
Louis: Do not follow me!
Bill's own internal monologue as he jumped off of the bridge is nothing but sheer awesome. It's less of a man leaping into a fight and more of a father intensely protecting his child. And he does this while charging headfirst into three tanks.
Louis' first encounter with a zombie ends with him beating it to death with a toilet paper dispenser.
As a Boomer, covering every survivor with bile and disappearing into the shadows without a scratch.
Finishing a campaign only using a melee weapon, you get an achievement for it and it feels good as well. Bonus points for the frying pan, katana, and guitar.
Keith. He may just be completely made up, but if he is real, then he's functionally immortal.
Louis's method of killing a Tank in the Sacrifice comic? Drive a fuel-leaking truck ramming into it and jump out at the last second, leaving a trail of gas for Bill to light him up. The others finish it off by exploding a gas can in midair.