The infamous Gnome with a Wand of Death is sort of the obvious next answer; in more general terms, anything on the Instadeath page that doesn't make logical sense. By that I mean that falling into lava without some sort of defense *should* kill you, etc.
At a more meta level, fixed quests and fixed time limits. I prefer the more open-ended and/or procedurally generated roguelikes (haven't played actual NetHack in years), and DF is sort of the ultimate expression of that. NetHack is a bit too much "puzzle game" for my personal taste. Everything in the world should have some internally consistent reason it is there, or at least *seem* to. ("Mad dungeon-obsessed trickster god" is a valid take, mind you, but that should have other ripple effects on the world.)
There's always
Elona, which is my favorite for it's utter lack of sanity checks. It technically
has a main quest, but you can feel free to ignore it forever, and it's basically just a dungeon you have to go a certain number of levels down, visit some quest characters for the plot, then return and beat a boss, while all the levels are still procedural. There are also some static instance maps with quests, but they're optional side quests. (Although defeating at least the first one is a good idea, since it provides a map that will never be altered, thus forming a safe "warehouse" for your excess wands/scrolls/whatever you don't want to either sell or carry everywhere.)
So far as the OP goes... It's a good thing NetHack lacks syndrome-bearing titans/forgotten beasts even on the first floor of the
cavern dungeon.