I'm not actually sure, and it's my favourite genre. I mean, a great plot is good, but it's rare for a story to be both excellent and open-ended, so a good story alone can't usually extend the lifespan of a game. The battle system should depend on how you set up your characters, but also have enough flexibility to require skill during the battles. For instance, the card-based system in Baten Kaitos depended so heavily on pre-battle setup, the actual fights were mostly about getting dealt the right cards, and trying to make the best of the damage multipliers availiable to you. It felt somewhat uninvolving. There should be a greater challenge than the final boss, to give you something to work at when you've burned through the main story. And it should make you feel suitably powerful by the lategame; if you're taking on world-ending demons that use extinction-level catastrophies as their default attacks, having a party member knocked out by anybody lower than a end-of-dungeon boss should make you feel indignant, not threatened.
Long story short: I love Disgaea. If a terrorist handed me a bomb remote linked to the Nippon Ichi studios, and a bomb remote linked to the entire southern hemisphere, and forced me to choose between them, I'd be seriously conflicted. Not about who to blow up, you understand, but because Disgaea 3 came out recently, so I'd naturally want to play that, but all the talk of bombs made me feel like going and playing more Makai Kingdom.