It's about when people make arguments for rule changes based not on balance or system issues, but just on the basis of what's fun.
Obviously, fun is subjective so I would really like it of folks would avoid badwrongfunning stuff. It's okay to say you don't think this or that would be fun, but don't attack posters over their preferences.
I am more interested in where conceptual ideas of "what's fun" intersect with design -- from the core system level to the individual adventure, spell and monster level.
Let's try and use examples, especially changes between editions or variant rules or house rules to talk about fun and design.
In games like D&D, especially the WotC editions, the fun is generally assumed to be some variation of being a badass and having an effect on the fiction...usually the bigger the better. Not necessarily winning with the press of a button, but that could be part of it, more that it's about fulfilling that power fantasy. So things like failing a check, missing, a spell not working, and a spell being counterspelled or resisted is less fun than passing the check, hitting, and the spell working.
But, weirdly, too much of that kind of fun becomes just as boring as its opposite. Easily succeeding and winning constantly is just as boring, if not more boring, than failing and losing constantly. I think when this stuff comes up it's more about the right balance of wins and loses to keep things interesting. It's not interesting to always win nor is it interesting to always lose. The interesting bits, to me, are the struggle and the challenge and the back-and-forth nature of the dice. The swing...the gamble...the anticipation of not knowing. The moment between letting go of the dice and the dice coming to a stop. Insert Willy Wonka meme here.
To me, 5E has gone off the deep end with character empowerment, meaning it's just always winning and that's the basic assumption of the game. Anything less than say 90% winning is seen, somehow, as "always" failing. I vastly prefer the old-school approach in games like B/X, AD&D, WFRP 2E, etc where your chances of success were much lower but, importantly, failing was also made to be interesting in its own right. Like climbing a wall. In AD&D, you're supposed to roll when the person is half-way through the climb. So you're half-way up a wall. Now you roll. That's tension. That's drama. Now most players and DMs make that roll at the start of the climb and the player nopes out of the idea if the roll is bad. How boring.
This is one of the things that bugs me about people saying they want drama and story in their games. Okay, so then why do you hate losing or failing so much? For there to be drama and story there has to be both up beats and down beats
. If it's all up beats, there's no tension, no drama, and no story. If you want drama and story your characters have to lose sometimes. And no, not maybe 10% of the time, far closer to about 50% of the time. That's roughly how most stories work. Robin Laws has written a lot about mapping story to RPGs. He's worth the read if you're interested in story in RPGs. Anyway...
Another example is Vancian magic. In AD&D, you have to pick the specific spell that you cast with that specific spell slot ahead of time. This limits the character, makes them less flexible, but it also forces players to get creative with their spell use and forces players to try to plan ahead. In 5E, you prep several spells and can freely cast any of them with any of your slots. This gives the character more options, makes them more flexible, but it also means they don't have to be as creative with their spell use and players only have to have a vague notion of what the day might bring. I prefer the AD&D style as it pushes creative use of spells and players having to think and plan. That's more fun.
Same thing with "encounter balance" in WotC editions vs TSR editions. The assumption now is that not only are all encountered balanced around the party but that all encounters should be fights and fights the party can, generally speaking, easily win. The assumption back-in-the-day was that the world existed independently of the party and that whatever the party encountered, it encountered. You roll up a wandering band of 100 goblins...then the party of four 1st-level characters encounters a band of 100 goblins. Hope they can hide, bargain, barter, beg, plead, run, etc...and the players had better be thinking carefully about how to handle that encounter or they'll die. Again, I prefer the AD&D style because that's more fun.