A combination of characters, plot and worldbuilding - sprinkled with a healthy dose of attractive artwork!
Those are ranked in order of importance, btw. I have to care about the characters to care about the plot, and all the awesome worldbuilding in the world (haha!) won't matter if the story and the characters in it are bland and boring.
Romance and good fight-scenes are fun too, but they're less important than character/plot/worldbuilding. Unless the romance is the plot, of course.