Absurdly High Level Cap: The game is usually beaten around the level 25-30 range. If there is a level cap, no-one has found it yet [1]◊.
Adventurer Archaeologist: The appropriately named "Archaeology" skill set revolves around being one of these. Not surprisingly, all of the skills are Indiana Jones shout-outs, and choosing the skill even starts you with a fedora in your helmet slot.
Attack Animal: This is the premise of the Golemancy skill tree, which allows you to summon Mustache Golems, giant robots, animated piles of kitchen cutlery, and immobile stone walls. That's not to say that other skills don't dip their toes into it, of course—Fungal Arts has slime and mushroom familiars for you to summon, Fleshsmithing lets you reanimate enemy corpses as Zombys to fight for you, Veganism and Psionics let you persuade enemies to fight for you, and Big Game Hunters can summon packs of trained hunting diggles.
Badass: According to the Left for Dred achievement, the player is this when it's earned. Considering that you need to beat the game on the highest difficulty with permadeath on, it's justified.
Big Ol' Eyebrows: Both protagonists. Lampshaded; monsters in the dungeon sometimes call you "the eyebrowed one" or taunt you with phrases like "Your eyebrows won't save you now!"
Bilingual Bonus: "Skål!", the shout that plays when you use the Lutefisk Horadric Cube, means "Cheers" in Norwegian. Lutefisk is a traditional Norwegian dish.
Booze-Based Buff: Liquor, in all of its forms, regenerates your mana.
Bucket Helmet: Buckets are low-level headgear. There's also plastic traffic cones.
Cartoon Cheese: At first you might think, looking at the wide variety of cheeses available in the dungeons (havarti, brie, blue, gouda, smoked applewood cheddar, parmigiano reggiano...), that this trope is averted. Nope! Out of all the different kinds of unique cheeses, there is one cheese known simply as "Cheese". It's yellow-orange and wedge-shaped, with holes in it.
Combinatorial Explosion: A literal case. Try and see what happens if you put a Horadric Lutefisk Cube inside another Horadric Lutefisk Cube! Or inside itself! (Made possible thanks to a hilarious programming oversight)
Crapsack World: Implied to be going on outside the dungeon; the Elves and Dwarves are dying out, after a war that used up most of the world's natural resources, big corporations push weapons onto the streets, the wildlife is dangerous and perverse, and universities offer degrees on going underground to most likely die.
Crate Expectations: The dungeons are littered with breakable crates (as well as urns and pots and so forth) that can include items.
Critical Existence Failure: Although the image of your face next to the Life Meter gets progressively bloodier as you take damage, there are no gameplay ramifications to taking damage until your HP hits zero (at which point, of course, you die).
This sandwich is of great stature; it has unmanned better heroes than you. Dare you feed on its glory?
A Date with Rosie Palms: How the wand-recharging skill of the Wand skill tree presents itself (and is treated in-universe, up to and including the disclaimer that it'll make you go blind if you do it too much). Which is actually true in this case.
Damage Over Time: Seen on many effects, including but not limited to Slimeburst Traps, Acid Burns, Curse of the Golden Ratio, Recursive Curse, and the special ability of the Golden Crossbow. There's also an even wider variety of Area of Effect spells that persist over several turns and deal constant damage to anything in range—these ones tend to be very effective for dispatching large groups of enemies.
A glance at the game's database files reveals crafting recipes, items, and even skill trees that did not make it into the final game. For the most part, just removing the lines that comment out the items isn't enough to bring them into the game.
An unintentional example of something being Dummied Out was the opening to the song "Brawl" (the Monster Zoo BGM). Until it was fixed in a patch, an error was preventing the opening from playing, causing the BGM to launch directly into the main "Brawl" music (the part that repeats until the Monster Zoo is cleared out). Another music piece, "Veil", may be suffering from a similar fate, as players have reported never having heard the song.
The Deadshot skill (which gave a handful of bonuses to crit chance and enemy dodge reduction, but nothing else) was removed from the game in a patch. However, it's still in the code, and can still be accessed by old save files that used it. The Steam achievement for maxing out the skill tree is still around too, but it's been rendered unobtainable—although there are workarounds.*
If you open the "last.txt" file in your Dungeons of Dredmor folder, you can manually edit the skills that show up when you select "Last skills" in the character creation menu. Change one of the numbers to 12 for Deadshot.
Elemental Powers: Many of the usuals like fire ("conflagratory"), ice ("hyperborean"), electricity ("voltaic"), and poison ("toxic") are here. And then there's some lesser known ones, like "necromantic" (seems to be the game's "darkness/shadow" element), "putrefying" (An entire element just for zombies and other undead), and "righteous" (light/holy damage). And then there's some seriously weird damage types, like "asphyxiative" (damage type caused by strangling and/or choking something) and "existential" damage, which, according to the helpful tooltip, may or may not actually exist! (It does.)
Empty Levels: Once you've maxed out your skills, you gain minimal, if any, stat gains from leveling up.
Exponential Potential: Take a few different schools of magic, and before long you can have more spells and skills than will fit on your hot-keys.
Game Breaker: The final spell in the Golemancy path, Digging Ray; "Busts through walls and is basically game-breaking," as described in-universe.
Game Mod: With the release of Realm of the Diggle Gods, Gaslamp added mod compatibility. Fans got busy very quickly, and there are now a wide variety of fan-made skill trees, item packs, and more. The official forum for game mods is here. Some of the more notable ones include:
Clockwork Knights: A skillset mod that gives several nifty abilities, bonuses to all crafting stats, as well as several new items. Notable for being the first mod to be officially endorsed by Gaslamp.
Interior Dredmorating: Greatly diversifies the types of rooms you'll find, as well as adding new items, additional name choices for named monsters, and new monster taunts.
Monster Menagerie: The obligatory "new monsters" mod.
Great Offscreen War: There are many references to the wars between the elves and the dwarves that have happened up on the surface, and the dungeons are full of relics from the conflict.
"Congratulations! You have died." followed by a gravestone for your character complete with what killed you and a short comment on your performance.
There are several achievements that require you to die to certain enemies, including Dredmor himself (which is very likely to happen the first time you face him).
Healing Potion: There's the standard Potion of Healing (restores 20 health), the Potion of Replenishment (restores 26 health and 30 mana), and the Potion of Lively Regeneration (restores 3 HP per turn for 12 turns).
Herd Hitting Attack: There are plenty of options; when you come up against a Monster Zoo, you'll want as many of these as you can get.
He Who Fights Monsters: The Demonology tree starts out with spells that are about slaying demons. But by the end of it, you will be a demon. It's worth noting that the first three skills increase your righteous resistance. It's when you start playing Amateur Solomon by summoning demons to fight on your behalf that the righteous resistance disappears of a sudden...then plummets. The descriptor for "No, You Are the Demons" even says "The abyss has not only gazed back, it's taken up residence on your couch".
Holier Than Thou: The Killer Vegan skillpath refers to its powers as coming from, in part, "moral superiority". The penultimate skill is actually called "Aura of Self-Righteousness".
Horny Vikings: Lampshaded with the "Historically Inaccurate Viking Helmet", which can be crafted from two plastic ingots and a Rough Iron Hjalmir (much closer in appearance to the helmets vikings actually wore.)
Hyperactive Sprite: All monsters continually walk in place or move their appendages, even when standing still.
Just Add Water: Whether you're smelting, grinding, mixing, beating, tinkering, turning or even distilling, crafting doesn't even take a turn. Even better, most of the crafting sets explicitly mention their portability in their descriptions, such as the disposable ingot press and the porta-still.
Katanas Are Just Better: The best sword class weapon you can craft is a katana. Lampshaded by its description, which says: "All the best stuff is made in Japan".
Lethal Joke Item: Plastic Platemail is surprisingly effective armor, especially if you're attempting to keep a high dodge rate.
A Loser Is You: The game absolutely delights in your death.
Lost Forever: As of the update that (among other things) changed the crafting system, the toolsets can no longer be used as extra storage space; anyone unfortunate enough to have been using said trick lost the loot within. Also, with the removal of the Deadshot skill, the achievement The Humanoid Typhoon is similarly unattainable.
"This Translation is All Wrong!" from the Archaeology skill tree allows you to re-roll the random stat buffs on an enchanted item, with diminishing returns if you use it too much.
Don't like the mushrooms you've grown? With level 3 in Fungal Arts, you can use "Mushroom Transmutation" to transform a stack of unneeded mushrooms into another random type of mushroom.
Alchemists can Transmute gems, if they have the wrong ones for their recipes.
Magic Mushroom: The foundation of the Fungal Arts skill. All mushrooms have magical abilities that grant various powerups. Well, except for the Mud Wen, which just poisons you instead.
Magic Wand: You can create them with the Wandlore skill, or just find them around the dungeon. Each type of magic wand has a number of charges to cast a certain spell; for example, a Coral Wand has a healing spell, a Bony Wand reanimates a targeted corpse as a friendly Zomby, and a Rock Wand shoots a giant boulder at an enemy.
Magikarp Power: Vampirism seems like a joke at first. Sure you regen health by doing damage, but you can not eat food, and you lose your natural health regen. That said, taking the second level in vampirism gives you the ability to eat corpses, giving you a potentially limitless food supply (and reducing the potential clutter in your inventory).
Mana Meter: Played straight, of course. Used for casting spells and stuff.
Mascot Mook: Diggles are doofy-looking drill-nosed bird things that fit this role quite nicely. In addition to the regular versions, they range from Angry Diggles to Arch-Diggles
Mind Rape: The Psionics skill is described as this, even earning the achievement "Get Out Of My Head" for mastering it. In practice, this is achieved through a basic hypnosis spell and a spell to make monsters fight for you for a time.
Money Spider: Most monsters drop zorkmids, for whatever reason.
The Monolith: One of several shrines involved in sidequests.
Suddenly The Dungeon Collapses: This achievement is our way of saying "thank you for participating in our voluntary quality assurance program."
Obvious Rule Patch: With the making of a wiki came crafting recipe spoilers. Obviously, this didn't mix well with the ability to craft things without having to know the recipe in-game, so now you have to search bookcases for secret recipes before you can craft them.
Odd Job Gods: Inconsequentia, Goddess of Pointless Sidequests. Oh, and the Lutefisk God (who is the God of Lutefisk).
Point Buy System / Class and Level System: When you start the game, you have a number of points to buy skill sets with. When you level up, you get to choose to upgrade a specific skill.
Poison Mushroom: Most of the mushrooms in the game are beneficial, but watch out for toxic Mud Wen mushrooms, which have no beneficial effects—they just apply a debuff. And if you're experimenting with unknown potions, there are several acids*
Aqua Vitae, Aqua Fortis, Aqua Regia, Acidum Salis, and Oil of Vitriol
that, despite being labeled "potion", are only usable as alchemy ingredients and will give you an acid burn if you try to drink them. (There's also a potion called "Verdant Poison", but hopefully that one is obvious.)
Purple Prose: A few items have some very overblown descriptions, but one item in particular really takes it to a new level. "Born of the gooey union of pre-born broodspawn from the stygian depths and befouled milk, this [item] slouches churlishly, just daring you to taste it. The item? A cheese omelette!
Spell Blade: Various buffs can enchant your weapons to deal all sorts of extra damage.
Status Buff: Oh, all sorts. There are buffs that wear off after a number of turns, buffs that require mana upkeep to keep going, buffs that wear off after a set number of attacks, buffs that wear off after taking a set number of hits, and even permanent buffs, which you can get by praying at a diggle god statue and won't wear off until you pray at a different diggle god statue.
Too Awesome to Use: Bolts of Mass Destruction are probably the most powerful weapon in the game, and are quite rare (if you find three by the time you reach dungeon level 10, you're very lucky; they are also hard to craft due to fiery wands being quite rare themselves). You can very easily "Not worth using it yet" yourself to death when you have one in your inventory. Most players save them for the Monster Zoos or Lord Dredmor.
Vendor Trash: If you do not have the relevant craft (Blacksmithing, Alchemy, Wand Lore, or Tinkering) or skill set (e.g., booze, etc. when you don't have a mana-using ability, or food when you're a Vampire, or meat/eggs/cheese/Dire Sandwiches when you're a Killer Vegan), many items are effectively Vendor Trash and can/should be sold without worry (or used in a Horadric Lutefisk Cube).