My main game does not do a very 'good job' distinguishing between Bosses and regular mobs. They are built using the same principles, for the most part.
This moreso means that regular mobs are built to feel like what most other people/games treat their Bosses as, design wise, than the reverse. This further complicates the act of answering the question, but the first thing I can say is that I only consider something a 'boss mechanic' if the game intends to leverage the exponentially higher TTK of the 'Boss' in some way. Otherwise I consider it a 'regular mob' ability or mechanic.
Interactions with the environment or special mechanics you must do to avoid a wipe or massive loss of capacity from the group are interesting, but they are designed to be avoidable. The most recent example is from Throne and Liberty's Kimer King (Chimera King? Kimera King? depends on your source).
The King uses many fire attacks and builds up heat over time as it is struck or attacks, similar to the standard FFXI behaviour (TP Gauge). When it reaches a certain value, it runs to the middle of the arena and begins to emit massive waves of heat, but it also is then standing under a cauldron of water or something that players can trigger by interacting with an environmental thing, to douse the King and prevent the attack. This is what I understand most games to mean by 'Boss mechanic'.
"Where I come from, you'd have to deal with it yourself." Basically. It isn't that I dislike these sorts of mechanics in any way, I just prefer that the tools to deal with them are part of someone's class kit rather than part of the environment
most of the time
.
I say all this to say that I don't distinguish well between 'Boss Mechanics' and 'Regular Mob Mechanics' outside of this, and I often expect that when someone says 'Boss Mechanics' in any 'specific' way, they mean something like the 'Dump water on Khimer King'.
Since, if they didn't mean that, they would be referring to what I consider 'regular mechanics', if a bit powered up or synergized better. This is no longer my expectation from MMORPGs (and should probably never have been, I'm just lucky to have landed on the one I would prefer the most).
Similarly, I don't think there are any mechanics in FFXI that I really find particularly unfun or frustrating entirely. Certain 'CC' style ones or Charm abilities can be upsetting because they force you to bring particular Classes but this is really just affecting your ability to lowman the content, which is on you as the player choosing to do that.
Discounting FFXI therefore, and also anything 'basically similar to Monster Hunter', which tends to rely on player execution and targeting skill moreso than statistics, I'd say my most fun bossfights were usually in Onigiri. Two are memorable for what they were specifically.
Tiamat was incredibly challenging/punishing at the time my group met it in story, not sure if we tried to do it at lower levels than expected or not (this is common enough that I can't discount it). It required lots of spacing and timing attacks and healing positions while constantly causing the battle to shift, in terms of whether any given player should be on offense or retreating. Very mobile.
Ahriman was interesting in an entirely different way. Due to either an Easter Egg or my 'glitch farming' ability, we didn't really 'beat' Ahriman, I just got the feeling that if I took hate and ran in circles around it for a long time, it would eventually 'get dizzy' and stop fighting, as it was basically 'a giant eyeball in the center of a room'. For whatever reason, it worked, but I expect that it was an Easter Egg because the fight is technically slightly more difficult to do in this way, and there were some signs that it was 'adapting' to this for a while.
Generally, my preference for bosses and mechanics in general tends toward 'too complicated to describe in any reasonable length post'. Other than that, it depends on the game, because what is challenging in FFXI is not the same as what is challenging in Monster Hunter or Onigiri, and I don't lump Fantasy MMO bosses into the same piles (except WoW and FFXIV, which as far as I know is the result of an intentional decision by Yoshi-P to cater to those who enjoy WoW style bosses). I can't comment on those two either because they don't interest me enough, though some of them do seem quite good at the highest levels, becoming more than just spectacle.
Other than those boss types, I'll try to think of anything that stands out as either 'avoid this because it sucks' or 'I always want a chance to fight this because it's so fun', but in general I don't experience either extreme very often in the games I choose to play.