Classic: UBRS. Too many people aggro the trolls. Too many people ignore the impalement quest. Too many people try to skip all of the bosses they can. Too many people think they can pull two rooms at once. Too many people who either pull too much or too little. Runs take way too long. Environment itself gets old incredibly quick and it becomes mind-numbing in a very not-fun way.
Burning Crusade: Heroic Sethekk. Fears are the best way to challenge the group, right? Nearly every single mob having a Fear makes it challenging and not at all cheap, right? Getting feared into other groups literally less than 2 yards away and then getting chain feared all because the tank dare to deal with more than 3 mobs at a time is fun, right?
Lich King: Pit of Saron. Timewalking says all there needs to be on the subject. Not a hard or time consuming dungeon. But the playerbase makes it take longer than a mediocre group doing LFD BRD.
Cataclysm: Throne of the Tides. Dialogue is awful. Bosses aren’t fun in any way. Erunakk’s fight has never been properly tuned. The gauntlet before Neptulon is always bugged. Always. The elevator is way too slow. Just. Awful.
Mists: Niuzao Temple. NO ONE target the Amber Weavers and prioritizes them or interrupts them. Never gets out of the amber shield or pulls them out of it. Never uses the gong or kicks you if you so dare as to try to shave a full 3 freakin’ minutes off the 2nd boss, because God forbid you take advantage of the demolishers during the big rush pull. GOD FORBID A RANGED DPS OR HEALER STICKS WITH EVERYONE SO MELEE AND THE TANK DON’T HAVE TO STOP WHAT THEY’RE DOING AND CHASE THE 3RD FREAKIN’ BOSS FOR AN ATTACK THAT DOES NEGLIGABLE DAMAGE AND KNOCKBACK. This is why people leave after the first boss.
Warlords: Mythic Skyreach for overtuning the living hell out of Ruhk’ran, and leaving it be for the rest of the expansion. Actual design and content goes to Everbloom for making unnecessary winds and turns to pad length alongside trash that was literally Subject Zero for the awful trash-centered mentality Mythic+ ended up being.
Legion: Actual content and design goes to Seat of the Triumvirate. Overtuned just like Cathedral, not a single thing about the dungeon stands out about itself other than remembering how people never could deal with the oozes properly. None of the bosses were memorable. An actual and serious chore. Personally, to me, 2nd place is Maw of Souls. Not a bad dungeon, but I have a deep seated and intense irrational phobias for deadly storms out at sea–and seeing a huge amount of water spouts with colossal waves means I have to focus on the floor the whole time. You can see where that leads.
BFA: Seriously and truly, all of them. Freehold’s bosses are either total silly memes at best, or downright unintuitive at worst (looking at you, Captains). Sethraliss is STILL bugged and likely will never be fixed. Korgus in Tol’Dagor is a nightmare for melee DPS that rely on extremely short CD periods to offload damage. That’s just the baseline stuff as an example, and isn’t including how utterly ridiculous it gets with Mythic+ and affixes. Don’t even get me started on MOTHERLOAD.