Anything that doesn't set the boss up as "A test of what you've learned so far".
I'm sorry, but I am perpetually annoyed when a game's combat system consists solely of me just using all my best skills and equipment and winning anyway... with some healing from time to time. This is even worse when it's a boss battle. I don't care what gimmicks you've put on your boss. Unless your boss DRASTICALLY changes the way I'm playing the game... I am not interested.
It's a snore-fest at that point. A waste of my time and yours.
For example:
The Spider Boss is immune to most every state you can inflict, but is weak to Fire, and it has a gimmick of making itself invincible for a few turns so you gotta turtle up... or it has a charging attack... or whatever.
All that will do to me as a player is... Force me to spend some unnecessary turns to heal up or defend myself before going back to spamming "Fire" every single turn.
--
I want a bit of a challenge. I want the game to teach me combat mechanics and the bosses to TEST ME on how well I know those mechanics. I want the boss to iterate on what I already know. I want what the boss does to slap me in the face and FORCE ME to do combat differently than I would in any other game.
In short, if my strategies for beating Final Fantasy 1 on NES translate very well into your RPG to ensure I win...
Then I'm bored and I don't want to play it.
I want my challenge in the form of making me play the game differently than I'd been playing it up to that point.
To that end... things that just annoy me about boss fights:
Immunity
If the only way you can think of to keep me from murdering your boss monster with Poison/Burn/Bleed is to make them IMMUNE or RESISTANT to these... You're not any good at designing a boss. In fact, you make me not want to play your game since you lack the imagination to even do anything interesting to the combat if the boss ends up with one of these states.
In RPG Maker, you can literally change the boss's ENTIRE MOVESET if you desire to do so (no plugins required!) once they are poisoned/bleed/burned. You can! You can even make your boss cure these states if you want to!
Seriously, if "immunity" or "resistance" is the only way you can think of in order to keep your players from steamrolling your boss... you designed it wrong. You need to go do some research on just what kinds of creative things you can do with your engine and get some actual inspiration on how to make your combat interesting.
Also, I hate bosses that are "immune" to things. Seriously, all you're doing with that is limiting my toolkit for dealing with the boss. Do you find that fun to have parts of your toolkit just rendered absolutely useless? I don't.
Stale Gimmicks
While we're at it, let's try to avoid gimmicks that some other game has already done before. Or, at least, once that every RPG doesn't already have as "default". I love me a good gimmick, especially if I have to experiment a little to figure out how to beat it. I don't like when your gimmick is something I already know how to beat the moment I see it. This is not fun for me. It is not fun for me, conqueror of 10,000 RPG, to play your game and fight the same boss I fought in the other 9,999 RPG's before this one. It's just not fun. I've been here. I've done this before. Can we at least get a new twist on an old mechanic? I have no desire to "turtle up" for big hits. I have no desire to "stagger" bosses. I have little desire to destroy adds before fighting the main boss. Just... do something where I haven't done this before. I don't want to know how to beat your gimmick the minute I see it. If that happens, I'm bored.
Hit Sponges
If your boss takes more than 20 turns to beat... I'm not interested. By "turns", I mean everyone in the party gets 1 action. If you have 4 party members, we're looking at 80 ACTIONS. This is excessive. I'm not interested.
If I wanted to be tortured to death by boredom, I'd just watch the "Twilight" movies.
Twenty turns is about all the patience I have. If your boss can't kill me in twenty turns, then I'm obviously too overpowered for this fight and my winning is a foregone conclusion. If I can't kill your boss in twenty turns, then this is dragging on too long, especially since I've likely already figured out how to win... it's just a matter of inflicting enough damage to win.
And with bosses like this being so common in RPG's, devs wonder why players like me go out of their way to grind a bunch of levels and steamroll their game. Gee, I don't know. Could it be I don't want to spend TWENTY TURNS IN COMBAT WITH YOUR BOSS? I don't want to waste my time after figuring out how to win and being in no danger of losing at all?
Dunno about you, but I'm not a fan of repetitive tasks that people aren't paying me to do. I work a job in real life. If you want me to do something so repetitive that it takes me 20 turns of doing the same thing over and over and over again... then you need to wire some funds into my bank account to compensate me for my time. At least my job has the courtesy of paying me for doing something boring and annoying.
Guide-Dang-It!
If your boss kills me and I learn nothing from that death... No. Just no. I don't mind losing a boss fight. I really don't. What I hate is when I keep dying and am learning nothing about this boss and how to win. This is a poorly designed boss. If I need to grab a guide to figure out how to kill your boss... you've failed. If I have to die to this boss an inordinate amount of times to figure out how to kill it... you've failed.
Telegraph your bosses properly. Sign post what I'm meant to be doing properly. Or, at the very least, reward my actions often enough that I can guess I'm on the right track and infer a smarter course of action after each death.
Basically...
Don't design bosses like Dark Souls. Design bosses like you actually enjoy people playing your game. Each death should mean something to me and teach me something new about what the boss does and how I should act/react accordingly. I should not see a boss do something and then have to guess 10 separate things in the vain HOPE I'll figure out what my counter to it is meant to be. I should have an idea of the possible counters I have available, and then attempt to use those in order to keep from losing.
Guessing games aren't fun. Never have been. Never will be.
Final Forms!
This goes hand-in-hand with the "Hit sponge" bosses. I don't want to beat your final boss twice. Or thrice. Or seven times, 'cause they have a bajillion forms. I don't. I don't care. These extra forms are basically never justified. They rarely, if ever, even add new challenge. What they usually do is just make the end boss take more hits to beat.
Stop it, please.
If a single form can't do the job of beating the heroes, there's no way a second form will do it. Or a third. Or a tenth. You're just dragging things out for no other reason than you have no idea how to create quality content.
Instead of adding another form, you know what you can do instead?
Have your boss monster change the pattern of their attacks in mid combat. Or... I dunno… add a new set of attacks as their HP gets lower, and they add more to the rotation.
Seriously, you don't need more final boss forms. If you can't make 1 form interesting, I seriously doubt you can make 2 or more forms interesting.
Bosses Without Challenge
Most every Boss I've ever fought against in an RPG is... well... just a DPS hitting me. I'm sorry, but that's not challenging. It's not even interesting.
You know what is interesting?
If your boss actively trolls me as a player. That's interesting. If your boss spends some actions to make me spend actions trying to undo what they just did to me. It doesn't even have to be deadly what is done to me. It just has to be annoying. An interesting boss is one that can troll your player and use all their skills against them. An interesting boss is not one that just delivers damage spikes and AOE hits.
Bosses like this are also far more satisfying to put into the ground.
I find it more interesting when every few turns a boss turns one of my party members against the whole party and I have to spend an action to undo the damage that party member did as well as another action to cure the "Madness" of that party member turning against me. It may not hurt me all that much, but it's a boss I'll remember as being annoying to deal with. I'll likewise feel elated at putting that boss 6 feet under for what they put me through in combat.
I don't find it interesting when I'm hit with a "nuke" or some other high damage skill all the time. I just heal that away and continue hitting the boss until I win. That's not challenging.
---
Okay, that's all I got for now.