For me, it's the plot, characters and depth that sell an RPG.
A well-thought-out story - not necessarily a complicated one, as long as it's told well - is worth its weight in gold. A deeply romantic story isn't always that realistic in a conflict-driven RPG plot, but a subdued or unique love story can be a nice addition, especially if the characters are endearing.
Characters should be diverse, have believable motivations and personalities, and shy away from stereotypes or simply conforming to what's considered 'cool' by a particular audience.
As for depth - by that, I mean a well-developed world with a tangible culture and back-story, not just a random assortment of towns and locations that have no apparent past, or reason for existing as they are. The plot, characters and world should all tie into each other - FFVII's a good example, with Shinra's military history, the role of Materia, and other such matters linking everything together.