What do I mean? At its heart a good sci-fi story actually IS another type of story. So, it might be an adventure, a romance, a drama, a thriller, or any other type of story, it's just dressed out with all the trappings of science and technology. The best SF tales, of course, would fall apart without their scientific foundation, but a purely scientific story is dull as dishwater to me...I know, I've read them before!
That's where my Mystery Principle comes into play (I just now made that name up, by the way). I don't think you should ever write a story without a little bit of mystery. You might also argue that you ought to have some Romance too, but I have found that if I strip away everything else, I at least want to be engaged in some unanswered question: a mystery.
I don't go out for actually sleuth-style or detective-style when I write, but I do want to make sure that there's something that keeps people engaged throughout. Right near the start, give them some unanswered question: an unopened box, a strange figure, a missing person, a secret destination. You want the reader to absolutely demand to know if everyone will discover the hero's secret identity, if they'll unravel the mysteries of the island they're stranded on, if they'll recover their missing memories.
A little bit of mystery makes sure that the reader doesn't turn the page because they want to, they turn it because they have to!