Doing It for the Art: Tarn "Toady One" Adams has described it as being his "life's work", reportedly turned down six- or seven-figure deals to license just the name because he doesn't want it to be associated with anything but his life-long creation, and the idea of working for the next twenty years to release v1.0 doesn't bother him. If that's not the very definition, then I don't know what is.
No Budget: The studio's annual operating budget is about US$35,000; for a game that's coded by one guy and which started out as a pure hobby project, that's pretty high. For a game that's won a large stack of awards, spawned at least three or four imitators from much larger and better-resourced studios, been the subject of a feature article in the New York Times, and now appears as part of an exhibit in New York's Museum of Modern Art, it's astonishingly low.
"Dorfs" for dwarves, "Dorf Fort", "Urist McX" for any given dwarf, "Cutebolds" for kobolds, "Hidden Fun Stuff" for gateways to hell.
"Cotton Candy" for adamantine, "Clowns" for demons, and "The circus" for hell.
"Party People" for ghosts, due to a rather infamous incident.
"Goblinite" or "Goblin Christmas" for the vast bounty of iron weapons and armors a Goblin Siege would bring, once your forces had killed them.
"Zombie Spirals" for a common occurrence when the player embarks in an evil biome that raises everything as undead. The more that get killed, the more undead there are to fight, until your dwarves are completely overwhelmed.
Throw It In: When Toady One was testing out the code for adventurer mode necromancers, he discovered he could raise a butchered animal's skin as a separate unit. He kept it because "it makes about as much sense as a walking skeleton."