Ah, fun. What a dangerous word. Is reading Finnegan's Wake fun? Is watching Schindler's List? Is playing Diablo? No, no, no, and yet we do it. Now, for certain, we have limited time in this world, and we want to spend it meaningfully, intentionally. No one wants to look back at free time spent poorly and wish they could have made another choice.
I'm going about this in this obtuse way because I've fought this fight too many times about video games. I've spent 70 hours in Getting Over It, a notoriously difficult and frustrating game, which many people felt was made just so that streamers could get rage clips out of it, but the reason I have that play total is because I've gone back over and over again. I've climbed that mountain over 100 times. I deeply appreciate what it is doing, and I've by no means mastered it, I still regularly encounter the issues that cause other players to scream that it's impossible. The entire game is a meditation on frustration, on set backs, on difficulty. 10 months before the release of that game, the developer posted on his blog
Eleven Flavors of Frustration. While I've seen it dismissed as trolling, much like Getting Over It, the more time I've spent, the more I see the value in what he's getting at. In Dark Souls and its related games, there's been a lot of griping about the need to run back to bosses after you've failed. It's tedious, it's repetitive, it's boring, it's a waste of the time. While I understand why they get that reception, there's sincere value to the approach. It gives you a break from the frenzied pace of those fights, allowing a bit of relief between the bouts of tension. It gives you a minute to let your last attempt sink into your brain, building up your mental model of the fight. It makes the challenge not about this one singular instance, but the entire process of getting there, so that you have bested an entire area when you finally get to move on. Elden Ring, by comparison, lets you spawn right outside the boss arena much of the time, and that's been praised in terms of "Letting you get right back into the fun part," I think there's meaningful quality that has been traded away in that concession.
Bennett Foddy uses the word flavor in that post, and I think that's a good approach. In the dish of free time hobbies, too much of a singular flavor, and you're going to get exhausted. Oops, All Horror can wear you out, no safe harbor to lean upon. Nothing but easy wins can be candy for breakfast, occasionally tempting but tongue numbing over time. Brutal challenge, unless you're uniquely determined, can drive people away who were once otherwise enticed by the meal.
So, we have to be careful when we look at the word fun. I don't think it implies the holistic nature that words like engaging, satisfying, rewarding do. At the end of the day, that's what I, and what I fundamentally think all players want to describe our time spent at these tables as, rather than just "fun". But I don't say all this to rob the word fun of its value, but rather, to emphasize its place within the context of the play experience, and how it contributes to the nuanced texture that leaves players actively wanting to come back.
So, to get back to the actual thing that was asked. One of the most uniquely praised aspects of TTRPGs is the freedom to do whatever you want (for varying degrees of freedom depending on the system). When looking at fun, in a way that can't be copied by other media nearly as well, I think that's a pretty great target. So, the opposite of that, negation, is a good place to start when looking at the most dangerous counterbalance. You can see the sparking idea here, Legendary Resistance, can easily come across as a complete negation of the players goal. When you try and hit a monster, you at least get to try and hit it, and see if luck is on your side, but LR lets the game just say that your action has become meaningless. Now, obviously, there's value to using up a resistance, and as I went into too much detail above about, value to not necessarily getting what you want. However, I think any area where a game is offering up negation as a response to a player's bid for agency, action, and freedom is one that needs to be handled with a very deft touch. A dash of it goes a very very long way, especially in the minds and memories of players.