And Your Reward Is Clothes: Each character will have alternate skins on the game's release, unlocked by completing in-game achievements. Until then, characters have a random chance of starting the game with their alternate skin during Early Access.
All There in the Manual: Most everything about the characters. Things like Steroids being a doctor, Fish having been a police officer and even a few character's genders are never mentioned during gameplay, or at best hinted at in the loading screen hints.
Awesome, but Impractical: Lightning guns. They fire a multi-hitting bolt of energy, but it functions much like lightning really would by routing to the nearest object. This means in narrow corridors it'll often arc away from your target and into a wall
Bling Bling Bang: Y.V. starts off with a golden revolver, in keeping with his "money to burn" characterization. Other characters can also find rare golden weapons, which have minor improvements over the originals such as increased rate of fire.
Body Horror: Melting, Eyes, and a few other enemy mutants.
Boring, but Practical: Melee weapons. Compared to the other, more unique weapons, they can be fairly underwhelming. But they can block enemy fire and kill most enemies that you encounter in one or two swings.
Bullet Time: Chicken's special power. Upgrading it gives her a normal rate of fire while using it.
Cast from Hit Points: The Rebel's minions are summoned for the cost of 2 HP each, and the Blood Launcher is an explosive weapon that uses your own HP as ammo when the regular ammo runs out.
Cast From Experience: The Horror can fire a beam of radiation that reduces his EXP bar. Upgrading this ability makes radiation containers hold more EXP, meaning more radiation to fire.
Damn You, Muscle Memory: Robot's special ability is to devour his current gun for ammo and/or health. This is incredibly useful, but if you had just played someone like Fish or Crystal who use their abilities much more liberally, you may end up chowing down on your gun in the middle of a fire fight.
The Dev Team Thinks of Everything: In update 25 of the game, Vlambeer added footstep sound effects, which changed based on what sort of surface players walked on. In update 27, getting the Extra Feet mutation gave you more footsteps.
Rebel has a B-skin which gives her a winter coat. What happens when you reach the frozen city with her? She puts on the hood over her head.
Drop the Hammer: There's a sledgehammer that can be picked up early in the game, and an energy hammer you can pick up later.
Dual Wielding: The unique ability of the Steroids character; all other characters can only have one weapon in use at a time.
Frickin' Laser Beams: Weapons that use energy ammo fall under this category, and the mutation Laser Brain increases the effectiveness of all of them.
I Love Nuclear Power: Radiation is the name of the game: all enemies drop it, and it functions as the game's exp system, with you being able to select a new mutation with each level.
One of the playable characters, Horror, is a monster made out of radiation.
Joke Weapon: The Screwdriver. It has short range, can only hit enemies head-on, as opposed to the sledgehammer and shovel, which hits enemies on a wider radius, and it's one of the weakest of the melee weapons. However, it can be used to repair a car in the scrapyard, which takes you to a mansion where you can find a golden weapon.
Kaizo Trap: The Dog boss of the scrapyard goes out in a series of spectacular explosions. These can kill you.
Also, if a next level portal appears next to a car, it can blow up the car while drawing you in, killing you.
The Hunter in the frozen wasteland loses control of his jetpack when you kill him, he can crash into you and explode.
The throne itself
Magikarp Power: Melting starts the game with 2 HP (For reference, a single bullet does 3 damage), but he gains levels much faster. If you can survive a few levels, he becomes incredibly powerful, especially if you can upgrade his special power to deal tons of explosive damage at no ammo cost.
More Dakka: Many weapons in the game have an automatic counterpart, which fires continuously as long as the attack is held, or an assault counterpart, which fires multiple rounds with each shot. Y.V.'s special attack also lets you fire two shots at once, and can be upgraded to four shots. Steroids can upgrade his special attack to get 20% of every shot free and make every weapon fully automatic, making the normal auto (and less accurate) variants of each weapon redundant.
One Hitpoint Wonder: Melting, in spite the fact he has 2 HP still is this due to the fact that everything (except touching maggots, being in fire for a short period of time, and a few others) does more than 2 HP in damage.
One-Hit Polykill: Crossbows can pull this off with smaller enemies, while the Heavy Crossbow will pierce through just about anyone.
Retirony: Fish was a cop one day from retirement before the apocalypse hit. He's not dead yet, of course, but this is a roguelike...
Revolvers Are Just Better: Every character has a revolver as their starting weapon, except for the Chicken, as mentioned above. Y.V.'s happens to be made of gold. Averted since these are their starting weapons and as such usually the weakest weapon in their arsenal.
Samus Is a Girl: Crystal, Chicken and Rebel are all female, but you'd never know that unless you looked it up. Well, there's the fact that chickens are Always Female, but even then.
Shotguns Are Just Better: There's a wide variety of shotgun-based weapons, ranging from the traditional kind, to a slugger that fires one giant shell, to a flak cannon that fires one shell that explodes into multiple shells on impact.
Shout-Out: the Disc Gun, Minigun and Laser Cannon from Super Crate Box make a reappearance, and Yung Venus from their previous game Gun Godz is now a playable character.
A secret exit portal in the sewer level takes you to a bonus room where the only enemies are four turtles and a rat.
Energy: Laser Pistol, Laser Rifle, Laser Minigun, Laser Cannon, Plasma Gun, Plasma Cannon, Ion Cannon, and Lightning Pistol/SMG/Rifle/Shotgun. Similarly to the Explosives above, the Energy Screwdriver, Energy Sword, and Energy Hammer are all melee weapons which use Energy ammo.