you kind of have to listen very carefully, or you will miss what the players are actually asking for
The big issue when people are somewhat resistant (putting it politely) to the idea that "fun" is a genuine issue that matters is that they tend to not what the players are actually saying, or even what other DMs are saying, in their rush to be sure to say that "X element isn't really necessary to fun".
So some things I'd mention, and this will include stuff already mentioned:
1) Spells/abilities simply bouncing off things for no particular reason.
It doesn't matter if it's balanced, or it's reasonable, it's not fun for spells to simply have no effect when the enemy isn't doing anything special to stop them (like emanating a shield or barrier).
Saving throws were a mistake. That's what I'm saying. They're much better suited to the wargames they came from than RPGs.
It's notable that few other games use all-or-nothing type effects like this, except those derived from D&D.
It's also notable that videogame CRPGs and the like rarely use anything resembling saves - I can't think of a single example in fact (outside of licenced or directly D&D-inspired games). Because they're not fun, however balanced!
Counterspell is sort of a subset of this. It's absolutely no fun for anyone except if a player manages to GOTCHA an enemy caster, but even then pfffft. It literally doesn't matter if it's a "good design decision" in your opinion, because we're listing stuff that's unfun, and it goes on the list. If you try and argue every point, there's not much point on the spot, making the list, is there?
2) PCs being incapacitated for extended periods
mentions this, and it's just bad. It's no fun for anyone involved. Not even the DM in most cases.
What's the answer? Asymmetrical design, essentially. Don't make monsters who can incapacitate PCs for more than round unless there's counter-play (like cutting off the tentacles that restrain them or the like).
3) Situations that force everyone to sort of stand around for an extended period whilst one PC "does their thing".
This is more of a Shadowrun issue or the like, but in D&D it can come up. There are ways to handle it, particularly with a strong descriptive DM, and letting the rest of the group know what's going on where it's less awful, but it can still be pretty tedious.
It's not fun when a battle's outcome has been decided, but combat rounds drag out for an inordinate period of time. It's not fun when a battle's outcome is determined within (or before) the first round.
Is not fun when too many real world problems cross into the game. It's not fun when the game has no connection to reality.
It's not fun when a game gets bogged down in tedious details. It's not fun when the game rules become too abstract and reductive.
All of this!
The combat one is particularly valid - a lot of otherwise-decent D&D DMs drag combats that are obviously "over" on for extra rounds, because technically that will use a few more resources. But it's often extremely unrealistic (cowardly bandits fighting to the very last man etc.), and extremely boring/unfun.
Real-world problems too, particularly the "everyone is wildly racist towards X PC" can get incredibly old incredibly fast.
4) All the NPCs are hostile wankers.
This is a classic "I saw a DM do it like this so I thought it was a good idea" piece of idiocy that even infects some professionals or semi-professionals, and videogame designers at the lower end of the design skill scale.
The PCs visit a town and just everyone there is nasty to them. Or like everyone in the entire world is. I cannot believe how many DMs I've seen try to operate like this. It's a huge mistake and very damaging to fun levels unless the PCs are total murderhobos and the DM is 100% okay with that.
5) Combats that last more than, like, 30-60 minutes at the outside.
Talking 5E here. If combat is taking that long, unless it's some epic final fight, I guarantee fun levels are dropping extremely rapidly.