Either deep, tactical combat, or deep character build customization. They usually go hand in hand.
A good job/class system will usually draw me to an RPG, while linear character progression will drive me away from it.
I heavily lean towards T-RPG though. FF tactics, Original Sin, Fire Emblem, etc.
For a J-RPG to impress me these days, it kinda has to spice up the combat. I can't really deal with old-school FF turn by turn anymore, I feel like it hasn't aged well compared to tacticals.
But at the end of the day if enough love goes into a game, petty preferences don't exactly matter. Great games don't necessarily catter to what players already like, they make them like something they usually wouldn't, instead.
DQ8's combat has everything I dislike from J-RPGs, yet it's easily one of my favorites due to the sheer scope and immersion of the world. Radiant Historia has one of the worst combat systems I've ever seen and its story shoots it above almost everything else.