Setting for science fiction is vast and diverse. I mean, Frankenstein could be considered science fiction, yeah? You've got hard sci-fi, space opera, post apocalyptic, utopian, dystopian, cyberpunk, steampunk, near future... the list is probably endless. To me, what makes sci-fi great is seeing how it affects the human condition. I tend to believe that people are people, regardless of setting, and what makes it interesting is seeing how people have adapted to or rebelled against the various sci-fi elements and how they retain or lose that humanity.
So yeah, as others have said, characters are the most important because that's who the reader is meant to empathise with, root for, or vilify. Next is the story, because what good are characters if they aren't engaged in a gripping plot?
Just my views.