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 and BioticsLab. 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.
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.
Crowning Moment of Awesome: From killing the Patriarch using the medic's MP5 to surviving a round where everyone else has died, odds are this has happened to you at least once.
Sirens. Their screams disarm explosives, cause exponential damage the closer you get, ignore armor, make your screen shake like crazy, and they have a good amount of health. 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.
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.
Sharpshooters still have a bit of this status, because the crossbow doesn't change its price depending on level - high-leveled sharpshooters spawn with it, essentially getting their choice of either a weapon capable of killing nearly anything with one headshot, or an extra £600 towards anything else.
Crawlers. GOOD GOD, 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.
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.
Everything the survivors say while handing out money.
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.