Apologies for this little ramble, but like I mentioned, it is quite difficult to articulate this properly.
For me personally, there are three aspects which specifically contribute towards what I would define as a great battle system within an RPG - strategy / depth, urgency, and fun.
The first aspect is incredibly crucial. I need the act of combat to actually be meaningful, to have a measure of depth which allows for the use of different tactics, or the consideration of different strategies, in order to win a battle. Not a simple - just attack and win 'one and done' approach. To this end, I love systems which present you with an ice-berg, allowing all users, of different experiences or capabilities, to see and touch a portion of the system, but if you want to dive deeper, they allow you to open the door and uncover a wealth of new possibilities, strategies, and combat potential. Ultimately, the strategic element within RPGs, for me, keeps combat from growing stale as you learn new abilities, figure out new exploits, or uncover optimum strategies for different encounters.
Some of the most highly-acclaimed systems have this ice-berg / rabbit hole approach.
Take Vagrant Story. You could easily go through the entire game without ever uncovering the hidden wealth of an incredibly detailed combat system. Or you could literally sink hundreds of hours perfecting its intricacies. Vagrant Story had a brilliant body-part attribution system. Targeting a different part of your opponent would have a specific effect - hitting the legs would reduce your opponent's speed, the arms their attack, the head their ability to spellcast etc... And that was just the surface. You could tailor weapons for each opponent type. Then there is the deeper rabbit hole of elements per opponent type. And we've not even mentioned skills and the 'risk vs reward' system that it incorporated.
There is also XII's gambit systems which allowed for a wealth of customization / automation. X's weapon attributes which tried to make every single member of your party useful in a specific encounter or situation. X-2 was just fast paced and fun.
For me, VIIR takes the best parts of all of these systems and combines them into a fantastic streamlined system that has great hidden depths for strategic options. Though nowhere near as detailed as in Vagrant Story, you are able to target some specific body parts of opponents, opening up some strategy and scripted elements in boss battles. It might seem like a simple thing, but spacial awareness suddenly comes back into play. Your distance from an opponent suddenly matters. What is between you an opponent suddenly matters. Environments can block you and your opponents moves, and likewise, if you are too far away from your opponent your moves won't effect them as well. Braver can actually miss! MISS! There is no mega-auto-lock-on here. This simple things throws up a lot of simple strategy in the back of your mind. You need to think about distance, about closing the gap, and switching to your other characters to make the most use of the battlefield.
I mentioned X becomes that weapon attribute aspect comes directly into play here. Cloud can't accurately hit flying opponents, or those up above. You need to switch to a long-ranger like Barret or use a Materia. This additional layer of strategy throws up so many options during an encounter, and the beauty of the quick character-switching option, with a flick of the D-pad, keeps the combat pace fast, and that sense of urgency immediate.
Then there's Tactical Mode. The ability to slow down time provides you with a breather and the option to assess your strategy - also allowing you to prepare a move with another character without even switching. This allows you to prepare multiple moves and combinations in order to optimally stagger your opponent and utilize your ATB for the full scope of your abilities.
So we've got the play of the battlefield, distance from an opponent and move distance actually being crucial, the ATB abilities, Tactical Mode, and character switching. We've not even touched on Materia use, Cloud's weapons stances, the other character's abilities, and Summons. Not to mention the other combat secrets that they haven't even revealed yet. There's so many options, which provides different possibilities and strategies within an encounter, which the short demo slice perfectly eluded to. What is amazing is how fluid all these systems and layers seem to work together. You can tell that a lot of work went into marrying all of them into one cohesive unit.
This is then combined with that sense of urgency that I mentioned was also crucial to creating a great combat system. You can have the best combat system, which can be intricate if you want it to be, in the world, but if battles feel meaningless then it personally drags the entire experience down. From my experience with the demo alone, that sense of urgency was upheld - that you are in an actual battle and the situation could go south very quickly. Enemies weren't a pushover, again harking back to the battlefield / range strategies, but also the Scorpion Sentinel, with its different phases and a far larger range of abilities, really pushed that sense of urgency. It wasn't just '+++ it's HP', it was an actual threat that nearly killed me several times, forcing me to remember to employ strategies, to hit the weak spots, to use Barret for the long-range assaults, to utilise Thunder, to use Potions and Cure Materia, to stagger then unleash the full force of Cloud's Limit Breaks ... I couldn't believe I was doing all this in a FF game, let alone my favourite.
Ultimately, I had a huge grin plastered on my face throughout the entire demo ... and long after I had left the play area. Those three crucial aspects of a great combat system - depth / strategy, urgency and fun - it had it all for me personally. And again, that was only a very small taste ... from a demo build that was 7 months ago!
If you liked any part of XV's system, then you will love this. It feels like another level. That's not even hyperbole there. The only missing bit of icing on the cake is the Materia Combinations, but we know there are linked slots, but it's not something that they've really highlighted yet. I actually hope they won't ... I can't wait to discover that in game. It was one of my favourite parts of playing VII - growing and messing around with different Materia combinations, and here it just feels like it will be just one part of a fantastic combat system.