That is a really hard question to answer. Do you emphasize the fantasy, or not? A really great story is going to be about awesome characters that you fall in love with. Beyond that, it’s going to need a really great plot. You can’t separate these things from writing a great fantasy, because while I think the worldbuilding needs to be really cool, if you have terrible characters and plot, it doesn’t matter how good your worldbuilding is—you’re not going to have a good story.
That said, the core of writing great fantasy as opposed to other fiction, assuming that you’re already doing the plot and the character right, is to get down to that idea of the sense of wonder. What is wonderful about this place that would make people want to live there, or be fascinated by it? What’s going to draw the imagination?
Fantasy is writing books that could not take place in our universe. For me, that’s the dividing line. In science fiction there’s the speculation “This could take place here,” “This may be extrapolating science beyond what we know, but it could work.” In fantasy we say, “No, this couldn’t work in our ruleset, our laws of the universe.” That’s really focusing on it is what makes the genre tick. So you have to do that well.