Balance and difficulty are important aspects for me when it comes to an RPG's progression system. Ideally, for me, if I was to play through a game without grinding at all, I'd want the difficulty to be quite high, and by investing time in leveling up or getting new gear, the game would fall to a moderate difficulty.
This is my problem with Pokemon games, which I enjoy playing, but I feel tempted to avoid optional encounters in order to maintain some of the difficulty. Pokemon Let's Go is a major offender here, where you level up so quickly it takes away from the experience (pun not intended).
FF15's issue was that battles could be cheesed. You could hold 99 Phoenix Downs with you, and you were allowed to use them on yourself after dying, so it made the game trivial from start to finish. A difficult battle turned into a question of whether or not I was willing to spend 15 minutes spamming Phoenix Downs or not.
In old school RPG games, MP used to be a precious commodity, and the only way you could recover it was by resting at an inn or using an expensive item. I like this. Persona 5 does well in this regard, where going through a Palace is a risk/reward based on "can I make it to the next safe room with my current SP". It had the difficulty and the balance as well.
I think a game's length also plays a role. Most of us like long games, or at least when it comes to RPGs. Investing time into developing your party is satisfying, and having long-term goals can make it really feel like you've grown as a character once you've reached the end.