Body Horror: Becomes increasingly common as the story goes on.
Bonus Dungeon: When you enter a building there's a random chance that it will have an entrance to a randomly-generated bonus dungeon where you can find tons of useful gear. The dungeon disappears once you complete it or if you leave the building without entering the dungeon, though. Choose wisely.
And when you encounter them, the background changes to a bloodstained complex with words written in blood ranging from threats to pleas for help. Fun stuff.
And it's highly likely you'll meet them long before they're properly introduced in the third chapter. You could just be walking along an alley when BAM POW MOTHERFUCKER GORE EVERYWHERE DEMONIC CHILDREN WITH KNIVES CRIES FOR HELP WRITTEN IN BLOOD.
Critical Existence Failure: Averted. You can't move as fast if you're low on health. Hunger slows you down. And fatigue distorts your game view.
More Dakka: One of the powerups in arcade mode changes whatever projectile weapon you're using into a spread gun version of itself, while still only using one bullet per trigger pull. There's also a rapid-fire powerup in arcade mode that...Well, makes your gun shoot faster, turning even the meager 9mm pistol into a good suppression weapon. Quite handy. The TEC-9 machine pistol and the Mini-SAW are mondo heavy on the dakka just by themselves.)
Musical Spoiler: Of the "basic beat with more instruments added as the situation worsens" variety. Not too spoiler-y since the music change occurs alongside whatever's happening, but oftentimes the first clue that the quiet, unassuming building you just entered is a zombie-infested death trap is the musical explosion that just deafened you.
Nintendo Hard: Safehouse takeover past level 5 or so. Anything in the double digits takes it Up to Eleven. Also, just crossing the street is hard in Act 3, even if you have a crew of people with mini-SA Ws. You'll have to stop at every other building just to make sure they don't run out of ammo.
For context, that's the last thing that particular zombie says before its head explodes and spider legs sprout from the neck, which continues to fountain blood as the body chases you around the room.
Too Dumb to Live: The friendly/neutral human AI is pretty stupid. It gets very annoying when the person your escorting decides to run off and fight all the zombies with a freakin' knife!
After you've taken over a few houses you'll start getting party members who carry machine guns instead of knives. They're so effective that you'll often find yourself outclassed and often unable to get many kills because of them.
Except when they intiate dialog, which renders them unable to defend themselves. They will initiate dialog in the middle of a zombie infested strret, during Act 3, at night.
Video Game Cruelty Potential: Punt around severed zombie heads like soccer balls! Destroy buildings with C4 to intimidate uncooperative building owners into ceding control to you! Light zombies and humans alike on fire! Influence people by pushing drugs on them! Run over zombies with a killer shopping cart! Electrically torture people with car batteries! And so on.
Walk It Off: (Slowly) regenerating health is one of the special skills that can be purchased from NPC merchants. The Doctor class begins the game with it as their starting skill.
Wide Open Sandbox: Take story missions! Fight against either the police or the militia! Or just run around and loot and shoot!
Weak Turret Gun: Averted. Sentry turrets and Tesla devices will save your bacon big-time, especially in the 2nd and 3rd acts.
Zerg Rush: When trying to take leadership in a neutral building the zombies will bumrush you through every available entrance. The strength of the zombies and how long they'll keep pouring in depends on the difficulty of the takeover.
Sometimes an entire horde of zombies can appear out of nowhere and swarm you.
Zombie Gait: Features both classic slow zombies and sprinting 28 Days Later-style "Infected", the former having been zombified for longer and therefore slower and further along in decay.
Zombie Infectee: AI buddies who are low on health and have been zombie-bitten will eventually turn into fast-moving Infected if you don't rest them at an Inn or give them medical attention.
Contrary to the trope, though, is that your buddies usually request you or another AI buddy to kill them before they turn. You can't actually attack them before they turn, but it's good to know you have their consent.