[rcxdude]

I think the main thing, stat-wise, that leads to un-fun is when there's imbalance within a party. It's dull to play a power fantasy game where the other players are significantly more powerful than you. Balance is the main concern.

Now, a good DM can house-rule around a lot of these things, but designing rules for balance is quite hard, as is learning new rules, which is why these systems are a thing in the first place, so ideally the rules should by default allow this kind of creativity and flexibility without creating large power imbalances, both between players and between players and monsters (also something that's more difficult than it looks, hence things like challenge ratings and pre-built adventures).

(I'd argue the fairly high variance of D&D combat also causes problems here, both for fun and balance, because it's no fun when a powerful character completely bricks in a fight against a lesser opponent because of cursed dice, and it also makes it harder for the DM to get useful feedback to balance encounters)