tbh the best part about a dungeon is the environment. the aesthetic is what makes it memorable for me. There’s a reason original Sunken Temple and Mauradon stand out to me as amazing…by today’s standards, they wouldn’t be labeled as amazing probably, because they took a while…i think they’re doing a good job appealing to both fantasy nerds and the speed runners tbh.
i think dungeons like freehold and were unique too…outdoor dungeons and letting us mount in certain ones always feels great 
Having just run Sunken Temple on retail on one of my pre shadowlands alts, I was sorta mystified at what the dungeon had been condensed down to from its days in vanilla/classic.
The dungeons in SL are more straightforward and not wide open areas with mobs haphazardly thrown everywhere like BfA
Fun floor-plans. Mob variety as stated before up there somewhere. Engaging boss mechanics. Diffult, but not impossible. Or, if not difficult, then at least fun.
Oh and the one that gets to me the most. No impatient, rude idiots int he party, that helps too.
agree 100%…i mean they didn’t even have to do that…a lot of that dungeon was optional bosses anyway lurking in the areas you didn’t need.
atmosphere and engaging mechanics.
a dungeon where you just kill wild animals is never that fun… a dungeon where you kill intelligent beings who deserve it, however…
Hard to really pinpoint. Its easy to list the things that make a dungeon annoying or unfun:
Camera angle restrictions are probably the #1 offender. Last boss in Maw of Souls is a great example, as are sections of Scholomance, Waycrest Manor and others.
Boss mechanics that are flat out annoying instead of interesting. This one is going to be personal preference more than anything else. IMO Inquisitor Tormentorum and Cordana (VotW) are 2 of the worst dungeon boss encounters ever created, while the 2nd train boss from Grimrail Depot is one of the best. Both have ways of punishing the player for lack of understanding the mechanics.
Low contrast aoe that makes damage tougher to differentiate / spot is another factor that can make dungeons annoying (Darkheart Thicket had this in some encounters).
As to the rest… fun dungeons offer room, bespoke areas that are visually impressive (this is more of a rarity in the game these days), boss kills that are woven into the story well, rather than placing the character in an area as just filler (De Other Side, for example)
TL;DR = I don’t know, but I know it when I see it