These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Killing Floor
Abridged Arena Array: Not as severe as it happens in other multiplayer games, but many games are played in WestLondon and Farm. Explanation: The easiest perk to level in the game is the Sharpshooter because headshots with the standard issue 9mm pistol count towards perk progress, which further gives damage bonuses to the pistol among other weapons. Both maps have plenty of open space for landing headshots before the zeds close in, and WestLondon is also among the brightest of the default maps. The most played custom maps, like ArcadeMall and MountainPass, are also wide open. Other maps that are played a lot are Wyre, a wide open map, and BioticsLab, a closed map that hosts no less than two easily defended killzones. Tragically, other fine maps such as FilthsCross and IceBreaker are almost always ignored.
And now that Mountain Pass is an official map, it's probably just as widely played as West London and Farm.
Base Breaker: The fact that DLC has been getting more expensive - it used to be two bucks for four player models in each pack, but since the release of Ash Harding has now escalated to five bucks for one. Weapon-based DLC has been hit with this too; for every pack actually adding new weapons, there's another one consisting solely of reskins for existing ones, not to mention they're even more expensive than characters.
To be fair, most of the five-dollar characters have some unique attribute, like a new voice pack (Ash Harding, D.A.R., Reggie the Rockernote although Reggie has far fewer lines than existing voicepacks - literally only one response for any given situation) or some sort of trail following them (money for Harold Lott, feathers for the Chickenator); the characters in the two-dollar packs just use the same voice lines as the default characters.
Breather Level: The minute-long breaks between rounds. On the easiest difficulty, it's a minute and a half. However, if you are far away from the trader and low on ammo and/or armor, the break might be quite hectic in itself.
The first wave spawns only clots and the occasional bloat, so one has to really try or play on Hell On Earth to get killed. Even moreso at perk levels 5 or 6, where one can plow through the round with the bonus weapons.
Sirens. Their screams disarm explosives, cause exponential damage the closer you get, ignore armor, make your screen shake like crazy, they have a good amount of health, and the game spawns them at about the same rate as Stalkers. Luckily, they're fairly slow... but that doesn't matter when one comes up behind you and unleashes a bloodcurdling shriek right in your ear - or, worse, you're playing as a Berserker and you have to rush two or more of them at once.
Bloats. They come with packs of weaker enemies and Interface Screw you, amplifying the damage you take and ensuring that you die horribly.
Scrakes and Fleshpounds. Both soak up enormous amounts of damage and are unstoppable once they begin charging towards the players. Because how zed health scales up with the amount of players, they are borderline unkillable without focus fire from the whole team.
Fan Nickname: "Ninja Vanish" - the act of spamming a lot of explosives in the same area in a short lapse of time, typically done by a neophyte Demolitions rapid-firing an M32. This is not only wasteful, as everything short of a Fleshpound is dealt with easily with 2 explosives at most, but causes the resulting smoke to linger far longer than usual, obscuring everything that WILL approach after that. The Firebug can also get this by spamming the flamethrower needlessly.
Game Breaker: Several classes have had or still have extreme advantages in comparison to the others.
The Support Specialist class was this for a while, and it's still pretty bad. It gets all kinds of bonuses to shotguns, increased grenade capacity, increased grenade damage, increased weight capacity, increased welding speed... it's a Super Power Lottery of sorts.
It's (long) since been nerfed by patches which greatly increased the threat posed by many specimens (namely Gorefasts, Scrakes, and Flesh Pounds). Two new weapons were then added to put the other classes on even footing with the Support Specialist.
The Level Up content pack introduced the Demolitions class, whose easily spammable, rather cheap specimen-proximity-triggered pipebombs could be piled up and used to instantly kill the blissfully unaware Patriarch. Since nerfed, with smarter specimen/Patriarch pathfinding and a special "diminishing return" resistance for the 'Patty himself.
For the longest time, the Sharpshooter perk kept winding up with this status, as its higher tier weapons were, on headshot, usually extremely powerful to the point of flooring a six-player serverSuicidal DifficultyFlesh Pound before he even got within 20 meters of getting a hit in edgewise and still have the firepower or attack speed to wipe out a good deal of the hordes of low priority targets, too. As of this writing, the lever-action rifle is the only weapon of the Sharpshooter perk's that hasn't been nerfed significantly yet.
The crossbow, on itself, broke the game for a long while until it was finally nerfed. Back then, it cost a mere £400, regardless of perk levels. That meant everyone could easily buy it by Wave 2. The damage it did was greater, and the weight speed penalty for being burdened was lower, making it the side arm of choice for every perk despite its weight. Yes, the ammo was expensive, but you could get half of your max ammo for free repurchasing it because it also sold for £400. For a while before the balance patch, the achievement that was granted for defeating the Patriarch using only Xbows, Merry Men, had been achieved by 10% of the playerbase on Steam, which is a lot considering it was meant to be One Of Those Achievements. Eventually it was shot hard by the Nerf Gun, as its headshot multiplier was almost nullified for the specimens it was meant to kill in the harder difficulties, and purchase and resale prices were raised to £800 and £600 respectively. This resulted in Sharpshooters themselves attaining this status again, since it meant they could effectively get an extra £600 towards whatever they wanted if they were at a high-enough level to spawn with the crossbow; this wasn't fixed until the Halloween 2013 update changed it so weapons players spawn with at level 6 now always sell for £225, regardless of their actual value.
As of Jan 2014, the crossbow is still ludicrously powerful when used by a sufficiently-leveled sharpshooter; by default, headshots with it do 4x damage, and the perk bonus makes it even higher. Considering that any other hit with it causes 300 damage (few other guns can match or exceed that without being explosives), a headshot for someone not a sharpshooter causes almost as much damage as a pipebomb, at a fraction of the cost. In the hands of a Level 6 Sharpshooter, it does nearly double what a pipebomb will. The high damage output is nerfed on higher difficulties against Scrakes and Fleshpounds, as a form of balance, however.
Crawlers, especially if you're a Berserker. Their obnoxiously short hitboxes can completely ruin your momentum, practically guaranteeing you've got to fall back and switch to a different, less damaging but ranged weapon to dispatch them, leaving you wide open to all the other specimens. Appropriately, Crawlers are, in fact, human-spider hybrid hybrids.
Eventually became less of a problem as longer (and slower) melee weapons were introduced and the axe reworked, so that a zerker can now hit crawlers more easily with those.
Stalkers come in a distant second, since they're invisible until they start attacking or you're a Commando. And they love to take advantage of this just to piss off players with slower-firing weapons - they will often mix in with other groups of specimens and shield them from your bullets.
Clots can be annoying; five or six can swarm you, leaving you victim to Husks or Fleshpounds, or worse, the Patriach.
Good Bad Bugs: Buying a dual handgun adds half of the maximum ammo capacity to your ammo pool. Selling one of the dual handguns does that too. For most handguns, buying and selling one of your dual handguns maxes out your ammo at a lower cost than simply purchasing the ammo, specially at higher Sharpshooter perk levels.
Hilarious in Hindsight: Players clamoring for an official female player model were told for years that it was impossible to do, at least without giving her an existing male voice, due to technical limitations. Funny how that worked out.
Nightmare Fuel: The game pingpongs between this and the below trope, especially with the Hillbilly Horror event. Most of the specimen replacements just make pained groans, which given their deformities and injuries, are completely justified. The few that don't, meanwhile, tend to be absolutely hilarious.
Scrappy Mechanic: The perk levelling up system. It's not too bad early on, but after level 3 prepare to spend upwards of 10 hours to reach the next level. Raising any perk to level 6 is an exercise in tedium. And then there are the perks with two levelup conditions. The numbers for door welding are ridiculous. And God help you hunting down 100K Stalkers before non-commandos kill them.
Higher difficulties on public servers. There is no way to prevent lvl 0s and 1s from joining them; they will inevitably join in the buy period of a late round, fail to reach the trader, cause the round to spawn 20% more zeds because of the increased party size, and die in 30 seconds. You can vote to kick them; but the round usually begins before this is achieved and the damage is done already.
Scrappy Weapon: Before the introduction of the Demolitions perk, the LAW rocket launcher was an awful weapon. Extremely expensive, heavy, not much ammo, and a very high chance to gib yourself due to a huge splash damage radius. Annoyingly, it didn't even do that much damage to "boss" enemies - Fleshpounds and Scrakes could eat a rocket and still keep moving.
Exacerbated during season events; he reads and studies the Pungeon Master's sourcebook for just such an ocassion.
Circus Patriarch (after a Total Party Kill): "I see you enjoyed the show as much as we did. You might say... that it was to die for."
They Just Didn't Care: Some community-made maps were added into the game through updates. However, the fact that they're community-made is obvious due to spelling errors in map descriptions and logos or the HUD visible in menu screenshots. Past that, these maps also tend to play the Patriarch's theme, normally only played when fighting him, at any chance they get.