For most people it's pretty GRAPHICS, closely followed by much sentimentality within the story.
For me, an RPG can be good if the game is a decent enough compromise between
character build options, tactical complexity, encounter design, narrative choices, puzzles and overal challenge.
Combat in RPGs usually sucks, so it typically falls to messing with the statistics, encountering varied monster designs and picking some choices with consequences, to keep me entertained.
I also like a good presentation, but I have some tolerance for primitive graphics, generic music and cheesy voice-overs. I don't mind bad, cliched plots or I wouldn't be playing at all.