A few things that make a dungeon fun for me:
- Layouts and traps that make sense. And not necessarily "oh yeah, it makes total sense that there'd be this many rooms in this dungeon," but just a rhyme/reason to it that I have a chance of figuring out, ideally without dying during the figure-out process. There's no reason CRPG dungeons can't have the same level of baked-in clues and logic that a pen-and-paper dungeon (ideally) has. If the only way to solve a dungeon is to die and then just not do the thing that made you die before, that's a badly-designed dungeon, gameplay-wise. (In real life I suppose it'd be good to build it that way, where people just die rather than figure it out, but we're not talking about real dungeons here.)
- Ambience, both in an audio and visual sense. I want to hear drips and chains and skittering rats and distant groans. Even better if these sounds are cues as to what's to come (and perhaps no more distant groans once you've killed everything.) And no brightly-lit dungeons, that is rarely not distracting to me. I'm always installing mods into Skyrim and Oblivion to make the dungeons a tad less cooperative on the lighting front. Otherwise I might as well be exploring a mall. (Which, don't get me wrong, I'm totally on board with a game where you explore an abandoned mall. That was one of my favorite levels/areas in Cyberpunk 2077.)
- Fear. I know we're talking about traditional dungeons here, but to me Alien: Isolation had a great "dungeon crawl" feel to it at times, and part of what made it great was the sense that you were never safe. I like wandering/random creatures so that, even if I have played before, I don't know for certain that there will or won't be a monster around the next corner.
- Variety. While there's definitely something to be said for "a maze of twisty passages, all alike," I like the feeling of progression that comes with a subtle changing of the earlier-mentioned ambience. A sort of "Okay, we're getting into older sections of the dungeon" or vice-versa, "we're making our way into the newer portions of dungeon." And perhaps more creatures, better traps, etc. One thing I love about dungeons in Skyrim is when after going through numerous standard stone tunnels with torches and bric-a-brac, it suddenly opens up into a mossy cavern with shafts of light beaming down and stalactite/stalagmite pillars and whatever else. Anything to break up potential monotony, really.
- QUALITY CONTROL. None of the rest matters if the dungeon is buggy AF. I realize this applies to way more than just dungeons, but it always feels more egregious to me in that context, because they're usually closed systems and thus should be easier to beta-test, in theory. When you're running around outside in the open world with countless options at your disposal, it can be forgivable for the devs to overlook something the player might do in that context, but when we're funneled into a very tight and specific area with only so many interactive options, it really sucks when those options are glitchy, or the areas have clipping problems, and you get stuck in a wall or a chain doesn't react or NPC's walk the wrong way, or whatever else.