A combination of gameplay, and story. If either of them suck, the game will suck, is one is good and the other mediocre, then you have a game that I'll like, but likely not replay, if both are good, you have a replayable RPG.
Xenogears is a replayable RPG, excellent gameplay and story(The combat system is the best in any console RPG so far), while its prequel Xenosaga, has a great story, but lacks in gameplay, so while I loved going through it once, I have no desire to do so again, at least until I forget the story.