A.I. Breaker: The AI has no idea how to handle two traps next to each other placed just far enough apart that they won't explode if a zombie is in between them. They just stand there, slightly shuffling back and forth until one expires or you kill them.
Boring, but Practical: Your starting semi-automatic rifle, when upgraded, remains an effective zombie deterrent up until the last showdown, when the sheer number of zombies finally renders its leisurely fire rate impractical.
Foe-Tossing Charge: If the zeds back you into a corner, unleash the rush move and shoulder-barge your way to relative safety. Watch out though, the blade-fist super zombie does this as well.
Foreshadowing: A large, muscular zombie is seen in the first level, jumping onto the weapon shop van, scanning its surroundings and then jumping away. You do not fight this type of zombie until midway through the game.
It is explained in the scrolling main menu/loading screen PSAs that these health packs are, in fact, soda. Very, very healthy soda. It is not explained, however, why zombies are still carrying this soda or why it jettisons itself out of their bodies when they die, though.
Hollywood Acid: Watch out for spitter zombies. They come in packs, spit gobs of damaging acid, and leave a large puddle of it on death.
Hyperspace Arsenal: In the vein of classic shooters, Jack and Scarlett can each carry every weapon and throwable in the game.
Land Mine Goes Click: Go ahead, it's not like they're smart enough to avoid it. You can upgrade them to be able to detonate more then once.
Lightning Gun: The shocker, when fully upgraded it almost immediately kills any non-super zombie its lightning directly touches, including ones behind other ones, and it does small amounts of damage to the zombies it bounces off onto.
Powered Armor: You begin the game with three different suits of experimental armor, and find improved versions in hidden containers as you progress. Each suit consists of torso, legs and arms and each component affords bonuses to two of three attributes: Strength (melee damage to zombies) Endurance (damage resistance) and Agility (movement speed). The endurance-focused suit, Armoshell, in particular makes the survivors look more like space marines.
Science Is Bad: It's implied that a biotech Mega Corp. is responsible for the infection. Partway through the game the survivors make radio contact with a scientist named Morden who works for them. He initially appears to be on your side, but the ending shows he simply wanted to use the survivors' immunity to create new super-zombies. Why? Who knows.
Video Game Flamethrowers Suck: While upgradable, it only does a fraction of the damage you'd expect. It does however, open the zombies up to further attacks.
The flamethrower is an interesting example. While it is terrible if you hold down the trigger, it performs greatly if used in tiny bursts, slowing down almost every type of zombie and killing them in two puffs if you have it fully upgraded. In spite of this, it's better to just use other crowd control weapons or explosives when faced with hordes, as those are faster. Not to mention that the only thing worse than a zombie is a zombie set aflame.
Who You Gonna Call?: The menu features a news ticker that alternates between updates on the meta-game and displaying PSAs from the "Department of Survivor Security."