FF12 has the only JRPG battle system that I find actual real fun. Let me explain. There are three principal properties it has that no other JRPG battle system combines, as far as I know. (If you know of one, please tell me so I can play it!)

1. Battles are part of the world.
2. Enemies are part of the world
3. Characters automatically fight easy battles.

First: battles are part of the world. There is no whoooosh effect ending in a separate scene. A separate scene hurts immersion and usability as well. In games with separate battle scenes, I’ve spent quite some time over the years inadvertently backtracking in confusing areas after a particularly long battle. I just forgot which direction I was going in the maze. Separate battle scenes are a bad idea now enshrined in JRPG tradition because of Dragon Quest.

Second: enemies are part of the world. You can see them coming. You can sometimes avoid them. You can gauge their numbers. Many JRPGs almost get this by putting generic enemy markers on the world map, ones that rush you on sight. Uh, no. This doesn’t count, because it still means you need a separate in-battle ‘Run’ option, and you don’t get that thrill of finally deciding to attack a new enemy type to see how strong it is. It just helps you visualise the random encounter rate for that area.

Third: characters automatically fight easy battles. It doesn’t matter how great and deep your RPG combat system is, most battles will involve you telling your whole party to attack the enemy until defeated.
A variation of this is to simply skip sufficiently easy battles, usually by making enemies run.

Those are the three basics, but there are a lot of other things that help FF12 be so engaging.

Bonus:

1. Programmable rules
2. Battles last less than 30 seconds
3. Real time battles
4. Buffs persist
5. Ability to choose enemies (and Enemies out of your depth)

Programmable rules: Not only should characters know to attack by default, you should be able to tell healers to heal when necessary, casters to cast when opportune, and fighters to attack patterns advantageous for certain enemies. Of course, this leads down a rabbit hole that starts with holding A to make everybody’s move be Attack and ends with a Turing complete scripting language. A lot of action JRPGs automate your party members with simple descriptions like “All Out” and “Run Away Screaming". That’s nice, but it doesn’t really provide enough control—even FF12’s ranked rules are not quite enough to make sure MP is used only as fast as it can be recovered.

Easy battles should last less than 30 seconds. Anything more and you don’t get that sweet, sweet feeling of liberating enemies from their XP one after the other, which is a lot of what makes grinding fun (or at least bearable). It turns into a dumbed-down strategy RPG, one that’s easy and boring. Note: 30 seconds includes load times; JRPGs of the optical-disc era have a real problem with this.

Real time battles are more fun as long as you still have the option to pause the action and issue complex orders. Active Time battles are even better, where speedy characters attack noticeably more often. Again, if you want to have complex, difficult battles, the game should be a strategy RPG. For example, FFX’s normal battles were easy and boring, but the boss battles were fun because they were a basically strategy RPG battles without a hex grid.

Buffs persist between battles, lasting a fixed number of seconds or number of battles. Per-battle buffs are just another bad JRPG tradition that few games think to change. They ensure that buffs are ONLY useful for boss battles. Yet again, the alternative is to stretch your battles out so long that you are playing a strategy RPG (this is why buffs/debuffs were useful in FFX boss battles).

Ability to choose enemies: You should be able to see and avoid enemies stronger than you–areas should contain a mixture of easy, medium and hard enemies. Sometimes, the barrier to progressing to a new area should be a high proportion of hard enemies. This is probably the least important point of all that I’ve listed, but it’s a nice touch when you once or twice you run into a shockingly difficult pack of monsters and get to decide whether to turn back or press on.

A lot of these properties are stolen from Western RPGs, and may be particularly stolen from MMOs. I refuse to play MMOs for fear of disappearing forever, so I don’t know. Maybe somebody who has seen the depths of WOW and returned alive can comment below.

Further bonus:

Final Fantasy X : The Bad Parts

1. Total disconnect between the good parts–bosses and story.
2. The bosses are too hard to beat without excessive grinding. (For me, anyway)
3. Normal battles are not fun (either too long or too simple, or often both).
4. Levelling system caters strongly to the obsessed player. It behaves much like a normal levelling system (but more annoying) until the end of the game: that’s boring. Only if you decide to grind early and often will you be able to exploit its subtleties at endgame.