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: Dwarf Fortress
Anticlimax Boss: Forgotten beasts and titans made of fluid tend to be this, since they fall apart at the slightest touch. Werebeasts tend to change into their weak, naked humanoid form at the worst possible time (they're only beasts in the full moon), often the very instant after their arrival gets announced.
Broken Base: The friendly teasing that occurs between tile users and ASCII users, or between those who prefer Adventure Mode to Fortress Mode, is occasionally mistaken for this.
Players that find Fan Nicknames fun and those who don't have been a straight example at times.
Demonic Spiders: The giant, venomous, web spitting kind. Also Wolves, especially in Adventure Mode, where they can ambush a hapless low-level traveler and kill them in a matter of moments, provided they're surrounded.
If you ever see a zombie giant cave spider, RUN. RUN AND NEVER LOOK BACK.
Elephants have been toned down greatly from the days of Boatmurdered, but unicorns have since gained a reputation as psychotic serial killers who brutally murder dwarves at random.
Pond grabbers are the new carp. Not excessively hostile but if you fight them they are terrifying.
In previous versions there were the elephants, then the carp (even Toady One thought he made them too strong). Now it's Giant Badgers. You also occasionally run into randomly generated enemies that are made of solid stone or even metal, and are as hard to kill as that would imply. To make things even worse, if the Random Number God hates you, they can also have a dust attack that in its most deadly form an Instant Death Radius with about the same range as your archers. Also in previous versions, a Forgotten Beast made of poison mist. Then, a forgotten beast made of any intangible material could only be killed by being encased on obsidian or ice. However, forgotten beasts made of such material are now laughably easy to kill; one punch will tear off limbs.
Although they're not strictly enemies, they deserve mention here: The evil biomes occasionally have clouds of ash or mist that wash across the map, and, just like a forgotten beast, poison your dwarves with a random syndrome. A few minutes after the new version was released there already were reports of particularly nasty clouds wiping out entire embarks before they had a chance to dig in. The worst of these will zombify your dwarves instantly and turn them against you. Your best bet in evil biomes is to seal yourself off from the outside world and never look back.
Particularly evil biomes have spontaneously rising corpses, which can easily fall into this trope both on their own, and as a result of the fact that all their severed limbs also rise up and create more enemies. The only certain way to kill them is, of course, by dismembering them, then crushing the remains under a drawbridge before they reanimate. Not even magma can kill zombies; it just makes them angrier and on fire.
Giant mosquitoes in the intial 2012 release deserve a mention here, as, even though they're no match for a trained dwarf individually, every time they show up they bring a hundred friends with them, and, even if you simply lock yourself underground to avoid them, they'll lag your game to high hell due to sheer numbers. Luckily, this was because of a bug that was fixed within a few days.
Giant sponges have joined the ranks of carp with the new release. They're not supposed to move, so you can't drive them away from your fort, but somehow they can charge at your dwarves and easily crush them by pushing them around (they're huge, so even a push can crush bones), and are immune to normal weaponry due to lack of body parts and blood. Also, they can spawn in rivers and lakes, too. Normal sponges are like this, but without the killer strength and they can spawn anywhere that has water (giant sponges only live in savage areas).
Giant keas, in fortress mode. Normal keas (they're small green/grey parrots) fly in groups and steal items, which is annoying but generally not deadly. Giant keas not only do that, but are far bigger than your dwarves, also fly in groups, and tend to kill your civilian dwarves if they meet them.
Ear Worm: the background music, a rather hypnotic six-string guitar piece by the game programmer himself. Its ability to get stuck in your head combined with the listener's inability to actually remember how it goes suggests it may be some kind of ethereal god music.
Ballistas. There have been reports of people holding off huge raids with just one ballista, and people going on rampages in Adventure Mode with just a ballista bolt.
Whips (blunt) and scourges (edged) have an extremely tiny area of impact and 5 times the attack force of other weapons. Compared to other weapon attacks, a successful hit by one of these will focus all the weapon's power (and weight) into a single point on the target rather than a wide area. This will nearly always instantly crush (whip) or sever (scourge) a body part, making them the best melee weapons in the game by far.
Fan Nickname: Dwarven lightsabers/hypersonic hammers (flails).
Weapon Traps are considered to be game breakers by some due to the massive amount of damage they deal. Likewise for cage traps, which unfailingly captures anything that isn't immune to traps, even megabeasts (and if you put a Giant Cave Spider web on it, it catches anything)
Economically, stoneware pottery. Fire clay can be gathered indefinitely and stockpiled near a magma kiln, which perpetually churns out crafts and large pots. Considering that stoneware can hold liquid without being glazed, you will never have problems with booze again. The value of stoneware crafts is comparable to obsidian, but are much easier to mass-produce; a few years of constant production will give incredible purchasing power.
In Adventurer mode, sneak and find a tile where rocks can be picked up that is as close as you can get to the enemy's range of vision. Then start throwing the rocks until he dies (or passes out, at which point you can walk up and slit his throat), without ever being seen. A higher Ambusher skill makes this better, of course.
Make a small room full of low-quality menacing spike traps. Fill those traps with practice spears. Attach them to a lever or repeater. Station a squad of dorfs in the room and watch their dodging, blocking, armor using, shield using, and weapon skills go up at a ridiculous rate. As a bonus, if any of them actually manage to get hurt, it gives your doctors practice, too!
Which is why, of course, every single person in the world tells you not to travel alone at night.
Most thieving creatures, such as rhesus macaques, raccoons, but especially the flying ones such as kea and buzzards, because they have far more mobility and are therefore harder to kill.
Magma crabs, only if you embark on a volcano though. They live in magma and are very resistant to bolts, and fire liquid basalt that will maim or even kill your dwarves. They will attack anything at every single possible occasion. They have More Dakka. They spend years in the magma sea or volcano and will probably never leave. It's not very difficult to protect yourself against them, though.
Quite a bit of cavern creatures, such as crundles, creeping eyes, but most especially hungry heads. Small, flying, come to your map as a lot, tend to get stuck in corners and scare dwarves, and harass dwarves like nobody's business.
Kobolds, that is if you don't consider them as Woobies. Steal stuff, generally annoying, sometimes deadly, draw attention from more important tasks? Check all of these.
Good Bad Bugs: These are the kinds of bugs that make DF the game it is. Magma sea vanishing? Oops, it's all being sucked down by a hole into HELL.
Thirsty? Oh hey, I'm on the beach! Oh, I can't drink salt water? No problem! When you fall into water, your clothing and external organs get wet with an item called a "water covering". It's never salty.
Due to a few of the particulars of combat and skill training, along with an overpowered bite attack, carp in previous versions had the ability to very quickly tear your people to shreds.
A good bit of the popularity of the game — aside from the whole ridiculously-detailed fortress-building thing — is due to various bugs, perhaps "misfeatures", that produce unintentionally hilarious results: for example, dwarves not recognizing that they are on fire before attempting to drink from the extremely flammable booze stocks. The developer's constantly updated progress log is a good source for these stories.
See the community page for a batch of examples that the playerbase named.
Pinching (yes, pinching) was ludicrously overpowered in previous versions. Pinching someone in the neck while they're sleeping could sever their head and send it flying across the room.
And of course, the nicely illustrative, 'Got rid of world gen crash during succession after death of prolific long-standing position holders with inbred descendants.'
A bug in the way cooking ingredients used to work, which enabled a cook to produce solid meals out of nothing but booze; this has since been fixed.
Due to a rather peculiar glitch that has since been fixed, ordering the construction of metal goblets would turn even the most valuable bars into iron mugs. Some reported that iron bars turned into gold, as something of an alchemical counterbalance.
Let's not forget the fact that you can throw things without having usable arms. Or legs for that matter. Apparently your character spits them out.
For the first few 2010 versions, it was possible for creatures, including your dwarves, to melt if caught in the rain in a warm area. This leads to one of the most ridiculous exploits ever in Adventurer mode: by going in and out of fire to melt all the fat in your body (but avoiding bleeding to death in the process) you become effectively fireproof.
Speaking of which, the Adventure mode also has several conditions where your controlled character suddenly shifts to some other creature. Overlaps with Game Breaking Bugs depending on luck and personal preference as you may get something stupid like a kobold or something horrendously awesome and nigh-unkillable like a bronze colossus.
More in adventure mode: the effects of walking with a crutch (speed becomes dependent on crutch-walking skill, which grows as you move) only cease when you drop a crutch, not when you put it in a container or throw it. This lets adventurers missing legs regain mobility without losing use of a limb in the process and sell back crutches they're already "done" with.
Water will freeze in cold environments, the resulting ice can be mined out like stone, it will melt if it gets too warm. This is all logical and expected. But constructions built out of ice will never melt. You can build a magma aqueduct out of ice if you so desire.
Normally, necromancers and other unnatural types make people around them increasingly suspicious with their agelessness until they're run out of town. So, from the devlog: "In bug news, the zombies in a necromancer's tower became suspicious after the necromancer failed to age, and he fled into the hills."
Today: Then I talked to one of [the resurrected severed limbs], and it told me that it was peasant. It was flattered but had no need of my services. I imagine its little fingers were shaped into the form of a mouth and they flapped back and forth while it spoke with a high-pitched voice. I guess there's still work to do.
Dwarfs ordered to remain inside (40d) or in a burrow (2010 and newer) will exhibit a few bugs. A moody dwarf will ignore this order when deciding on what workshop to take, and will bring items to said workshop even if it's outdoors/out of the burrow. One still-standing glitch causes a moody dwarf to forget whether or not he brought items to a workshop that's outside of the area he's supposed to be in. Cue the creation of Planepacked, which took over a year for all the materials that ended up being used to be gathered and contained 73 images of itself.
A bug that briefly appeared in at least one version caused a sort of Goblin Civil War. Somehow, goblins were divided on whether or not they were loyal to Non-goblin leaders of goblin civilizations. This caused Goblin ambushes and sieges to immediately start killing each other as soon as they arrived on the map.
In one version of Adventure mode, sleeping on the beach may result in you being ambushed... by fish that immediately drown as soon as the battle starts.
Body Surf: Before 34.11, it was possible to suddenly shift from controlling an adventurer to an underground creature (even a demon), then to bodyswap to some other animal every time you go to sleep... and they still can talk. Humans are okay with a talking elephant.
When a caravan appears and you have a hospital zone designated, your dwarves will sometimes pour out of the fortress en masse, steal all the cloth the merchants are carrying, and take it to the hospital to use as bandages.
Memetic Badass: Toady himself. When you look at what he created, it's not all that surprising.
Any fortress will create a few of these if it survives long enough, but some are truly exceptional, such as the backpack-wielding dwarf of Syrupleaf, or the dwarves of Bronzemurder.
Captain Ironblood who never bathes (and is thus literally covered in blood, mud and vomit at all times) and can kill titans and dragons on his own. He eventually took up seige weaponry use as a hobby.
Cacame Awemedinidae, The Immortal Onslaught, Elf King of the Dwarves. According to the (partially player-invented story), he joined the Dwarven military after his wife was killed and eaten by other Elves, just so that he could kill more elves. He proved to hate elves so much, that he was made the Dwarven King.
Memetic Mutation: "Person cancels Action, interrupted by Thing", "Person cancels Action, taken by mood", and "Person cancels Action, too insane".
Migrants with near-useless skills, such as cheesemakers, small animal dissectors, trappers, etc.
Scrappy Mechanic: The economy in 40d. There's a widely recommended option to turn it off, and several other options to turn it down if you do permit it (none of which allow you to convert a stack of coins from a blindingly spammy list of one-coin objects.) DF2010 just scrapped the whole thing until Toady can make something that isn't horrendously broken.
The inability to designate staircases, item properties, or tree clearing across vertical levels, although this has been fixed as of the February 14 release.
Squick: Legendary wrestlers. Full stop. Even with the patch that reduced their effectiveness somewhat, they can still reduce a perfectly-functioning combatant in full platemail to a pile of gelatinous mass of broken bones and ruptured organs. They even seem to take time to break most of the bones instead of just choking the target unconscious and kicking them to death.