Zombies armed with hatchets or claw hammers can kill your survivors through the barricade in the second game, and in Union City, they hit really hard.
Military and police zombies have helmets and bulletproof vests that turn your attacks into Scratch Damage, if they don't No-Sell them completely. When they show up, your best target stops being the head and starts being the legs.
Aftermath has the Chargers. It's not that hard to dodge their charges if you see them coming and you're in an open area. IF. If you don't, say goodbye to about three bars of health (you max out at seven, by the way) and enjoy your VERY significant amount of stamina damage. They're also complete walls of HP with or without the body armor they'll start wearing once you reach the final sector.
Additionally, the final sector is home of the drone guns you may have seen notes about throughout the game. They can be destroyed, but if they notice you first, they can cut you down in seconds... if you're THAT lucky.
Game-Breaker: The UMP45 and Hunting Rifle from 1 can easily break the game's difficulty in half once you've got ahold of them. The former is a Disc-One Nuke that can be found as early as Night 4, where its fully-automatic fire will shred through the shambling hordes and make short work of even fat zombies, and remains viable up until the end of the game, even after finding the Uzi, M4A1, and AK47. The latter is a One Hit Poly Kill monster that basically runs on infinite ammo despite its single-shot nature, given how short the "reloading" animation is, and only narrowly avoids being Disc-One Nuke due to being found during the latter half of the game, but it still demolishes entire mobs all the same.
Zombie dogs are fast and a nuisance but deal very little damage to the barricade in the first two games, and only moderate damage in general in Union City.
Armored zombies downgrade to this in Aftermath. They're still immune to bullets while it's on and you can't normally deal damage to their bodies until it's off, but fire, explosions, and armor-piercing guns ignore the armor, and both melee attacks and bullets will slowly strip it off. Armored Chargers, however? See above.
Good Bad Bugs: It's possible to lose survivors on search parties at random. This can still happen when you have no other survivors with you. Basically, the one survivor you have (yourself) can die on a search party, possibly multiple times, and still go on slaughtering zombies.
In Last Stand 2, changing to your secondary weapon as the level starts gives the secondary weapon the same ammo count as your primary. If the secondary weapon is, say, the Hunting Rifle, which usually only holds one bullet, and the primary is an Uzi, which holds 50, the zombies don't really stand much of a chance.
In the first game, if you switch to your other weapon while firing an automatic weapon, the other weapon will have the first weapon's firing rate, even if it is also an automatic weapon. This makes the slower-firing but harder-hitting Magnum revolver more effective as the UMP 45 and it are the first two weapons found and the chainsaw counts as an automatic. When used on the pump shotgun or M82 Barrett, the game becomes substantially easier.
The death scene involves your character being tackled to the ground and eaten alive by zombies as he visibly screams and tries to fight them off. Only lessened by the Have a Nice Death message.
Some of the notes in Union City can count too. For example, there is a very poorly spelled note from a little girl whose nanny told her to leave a goodbye note to her mother.
Some of the Junk items in Dead Zone can do this to. One of the junk items that can be found are Teddy Bears, whose description says it all - "A child's stuffed animal that has seen better days."
The fact that some of the containers the survivors in Union City and Dead Zone can loot are fallen survivors, who obviously fell to the infected, or some other horrible mishap.
Nightmare Retardant: Many of the notes were community submitted and contain high levels of Narm.
Polished Port: The Legacy Collection rerelease of the original trilogy brings it to modern systems in higher quality and somewhat improved performance, alongside achievements and fullscreen play.
Scrappy Mechanic: The traps in the second game are impossible to completely control once the night starts. While the propane tanks can be sort of managed depending on your aim (which isn't guaranteed given how many zombies can demand you aim at them), the land mines and bear traps can't be manually used at all, only randomly triggering if a zombie decides to walk on them. Over the course of the game, it's guaranteed that any attempts to delay armed zombies will be denied because a runner decided to sprint over a bear trap or land mine and waste it.
The propane tanks and land mines, being explosive traps, will cause any other traps of the same type to also detonate when they do if they're on an adjacent square. This forces you to place them at least two squares away from each other, which severely limits your potential to control the explosions.
In the first two games, when the barricade is close to breaking completely, any survivors in your team will suddenly stop fighting and run away. On top of coming across like abandoning you at the last minute, you only have a short window to deal with any zombies that slowly close the gap. The instant one touches you, you die and have to start over, even if it's at the very end and there's only one zombie left.
Scrappy Weapon: The starting pistol in the original duology. Despite its sizable magazines, the pistol does barely any damage even with headshots, thus necessitating constant clicking to kill even the slowest of zombies, even more if they're fat zombies, which makes the early game absolutely miserable until the player have found better alternatives.
Sequel Difficulty Spike: The difficulty step up from 1 to 2 is quite significant. The first glaring change is that your barricade has considerably lower max hitpoints, which rises and drops depending on the location you're in, but generally hovering around the fifty mark, making any HP loss that much more punishing. Not helping matters is how much more zombies there are early on, and the types that were encountered around the halfway point in the first game (e.g. dogs, running zombies, armed zombies, etc...) can appear as early as Glendale, and they can now kill your survivors from outside the barricade, thus complicating matters logistically during the day. All of the above combined makes it very easy to snowball into an unwinnable situation if the RNG is feeling fickle with survivor or weapon finds.
Tear Jerker: In a meta example, with the end of Flash, the first four games will soon be no longer playable online after December 31, 2020. Players of Dead Zone have been telling each other their respective Good-Byes, and where else they can find one another. However, the developers released The Last Stand: Legacy Collection on Steam and GOG in 2021, which includes the first three games.
Unintentional Uncanny Valley: In the original trilogy. Doesn't apply for the zombies, as they naturally look freakish and ugly due to being undead. But the somewhat primitive graphics style ends up making some of the survivors (outside of Jack) look very ugly and abnormal, though you only really notice it if you go out of your way to look at them when defending.
For the film:
Common Knowledge: Ray Owens is depicted in much of the promotional material with a Smith & Wesson 500, implying it to be his weapon. In actuality, it's Dinkum's gun and Owens only uses it to dispatch Burrell. Ray's actual sidearm is a more sensibly proportioned pistol.
So Okay, It's Average: The general consensus of the film is pretty much "it's nice to have Arnold back, but it could have been better".
