So what makes kingdom hearts 2 final mix so good? Is it worth buying the ps4 remaster just to play it?
Honestly, it's just a really fluid system that offers a lot of mobility, a good amount of nuance when it comes to defense, and does a good job of emphasizing skill over stats.
Leveling up in the game primarily just increases either your maximum health or gives you abilities that mitigate the damage you take. The tools you have access to with regards to movement, blocking/evading, and offense are gained as you progress through the game, so playing at a low level doesn't limit the amount of depth to the battle system.
Also, there's a minimum amount of damage that you can do to a boss, so even during a level 1 playthrough you're never doing chip damage. Even with the hardest bosses in the game, you're never going to be in a position where it takes more than 3-5 minutes to take them down, and that's not because they're easy. The really good bosses, like the extra humanoid ones, are difficult because they are incredibly aggressive, and they aren't the type that will repeatedly alternate between a stage of pure aggression and obvious vulnerability. That's not to say that there are not openings, but the way the combat system works is that your attacks can stagger enemies. Your goal is to take advantage of the first opening in their combo that's large enough that you can stagger them without getting hit, and to capitalize on that as much as you can before you hit a hidden value in the game that allows them to retaliate. I'm not going to say it has the depth or skill ceiling of a proper fighting game, but I feel like that system provides a similar sort of ebb and flow to the combat.
Picking Critical Mode makes the game A LOT harder, but in exchange you're given an abundance of Ability Points, so you can have all the moves you want equipped at the same time.
Your defensive/evasive options include blocking, parrying, dodge rolling, dashing, dodging in the air, gliding, and reflect magic. I don't want to drone on too much since I've already written out this much, but what I would say is that there are upsides and downsides to each maneuver, and there's a good amount of thought put into the limitations of each one. When you're aiming for mobility with gliding, your access to i-frames is significantly limited, and the viability of reflect magic, for example, can be diminished depending upon when you do it in a combo, and it doesn't actually deal damage back to the enemy until you allow yourself to be vulnerable again.
The gameplay does have its shortcomings, and if you want to compare it to something like Bayonetta or God Hand it's easily going to fall short, but I feel like KH2FM shows a lot more understanding of what makes action games challenging and rewarding than something like Ys: The Oath in Felghana(even though I enjoyed that game) or almost any other RPG I've experienced.
I would suggest playing on Critical Mode at some point, but it's fucking hard so be prepared for that.