Big Creepy-Crawlies: Some of the aliens are vaguely humanoid, but most fall under this, more closely resembling spiders, slugs, Zerglings and who knows what else.
BFG: The minigun certainly qualifies, but first prize goes to the Magma Minigun, the game's ultimate weapon. It can clear the entire screen of lesser monsters in three shots (which is under half a second). Its ammo comes in stacks of 400.
Bloody Hilarious: This game doesn't seem to think much of the gratuitous violence going on.
Body Armor as Hit Points: Averted, armor pickups absorb a percentage of damage. Green flak vests absorb 50%, yellow combat suits absorb about 75%, and red Powered Armor absorbs 90%, in addition to making you look like a human tank and making all your guns freaking huge. The sequels add more different types of armor with various protection levels, which can be repaired for far cheaper then buying new ones.
Boss Fight: Though the first game relies on Multi Mook Melees, the sequel and the Zombie Shooter games add big nasty bosses for you to fight at key points. Zombie Shooter's final boss can only be harmed by luring him into an electrical trap. Zombie Shooter 2's final boss has a protective barrier that can only be dropped by destroying the generators scattered about his arena.
Cool Guns: Appearing in the sequels are the MP5, AKM, M14, RPG7 and others.
Cyborg: Your character can be upgraded with cybernetic implants, increasing health, strength (which determines how much ammo you can carry) running speed and accuracy (which also provides a small damage boost). The sequel also adds an intelligence stat, which determines how effective said implants are.
Doom Doors: Old-school shooter fans may recognize the use of Quake II's super shotgun firing sound for this series' shotguns. In addition, the engine and driving sounds for the vehicles seem to be that of Half-Life 2's dune buggy.
Dual Wielding: The first Zombie Shooter starts you off with a pair of what appear to be Enforcers. Your character fires them sideways with surprising accuracy and a respectable fire rate.
Emergency Weapon: You begin the game with a pistol, or Uzi in the case of Alien Shooter: Revisited. It's good against the small packs of Zergling-like enemies you encounter early on, but it's outdated by the time you find the shotgun.
Energy Weapon: You have the slow-firing, but powerful Plasma gun as well as the Magma Minigun. In later levels, aliens with Arm Cannons show up. The sequels give us a laser Sniper Rifle, the slow firing, but heavy-hitting Ion Cannon, the fast and deadly Laser Chaingun, and so on. Zombie Shooter 2 even adds a Shrink Ray!
Engrish: The briefing texts suffer from this. They seem to have gotten a better translator for the Zombie Shooter games.
Face-Heel Turn: In Alien Shooter 2, General Baker wants take control of the alien hordes for his own ends. He doesn't react well if you decide not to help him.
Firing One-Handed: How your character wields the starting pistol/Uzi, and every pistol save the dual pistols in Zombie Shooter 1. Perhaps if he/she used two hands, or two Uzis, it would be more effective. Every other gun is used two-handed.
Hammer Space: And how! The first games only let you carry a few hundred rounds of ammo, but the sequels really went overboard - you can carry several thousand rounds of various types, and if you spend some skill points in the strength stat you can carry several tens of thousands.
Hand Cannon: some of the late game handguns deal far more damage than a pistol's got any right to.
Heal Thyself: Available in instant-use pickups and storable medkits that automatically trigger when your health runs low.
Impossible Item Drop: while there are a few gun-packing enemies in the game, even unarmed ones who attack with claws and teeth will occasionally drop ammo pickups.
Kill It with Fire: Flamethrower? Check. The sequels also add the short-range "burner" weapon.
Large and in Charge: The alien commander in Alien Shooter 2 pilots a Humongous Mecha that dwarfs anything his grunts use. General Baker on your side arguably qualifies, what with his massive combat armor.
Ludicrous Gibs: Available in red or green! Any alien or zombie hit by a heavy enough attack will gib, sending parts flying everywhere. Expect the halls to be plastered with the stuff when you're through.
Mook Maker: Aliens can infinitely spawn from certain openings in the walls; in such occasions, you must deal with them with dynamite. Teleporters will also spawn aliens indefinitely, but they'll blow up with sufficient firepower.
Money Spider: Enemies occasionally drop wads of cash, but these are more often found in crates.
Power-Up: It's very rare, but an invincibility power-up can occasionally be dropped from an enemy.
Rare Guns: The first game's shotgun is a Pancor Jackhammer. In-game, it looks more like a Benelli or Mossberg pump gun. It reappears in the sequels as a mid-to-late-tier shotgun.
RPG Elements: In the sequel. You get to choose a perk, such as regenerating heath, increased experience gain, more money drops or the ability to have secret areas pointed out to you. This perk is upgraded with attribute points like your basic stats.
Serial Escalation: Just when you think the game can't possibly throw more aliens at you at once, it does.
Sniper Rifle: Zombie Shooter 2 adds sniper rifles, which, while slow firing, do more damage in a single hit then any other gun in the game. This includes the nuclear rocket launcher. You'd think the rocket blast hitting multiple enemies would even it out, but sniper rifle bullets go through anything that isn't solid level, so you end up carving corridors of empty space within the swarms of enemies.