Depends on what type of sci-fi we're talking about. Are you doing soft sci-fi or hard sci-fi? What genre of sci-fi is it: space opera, cyberpunk, post-apocalyptic, steampunk (God forbid), time travel, or any other such type? These variables will influence the factors of what makes good sci-fi.
I will argue that, at minimum, a sci-fi movie has to have all the qualities that make good movies in general, plus concepts that are rooted in at least some basis of scientific fact, theory, or hypothesis, and, if we're dealing with soft sci-fi, relevant, non-preachy social commentary.
