sci-fi is short for science fiction. Sci-fi is basically a fantasy which deals with the imagination of a different world which may involve creatures from different universe or aliens.
1) balance between reality and the sci-fi world.
Whether you are writing science fiction or not but for being a writer it is very important to balance between reality and the world you are creating. Because if you totally forget about putting the reality in your fiction then the audience will not be able to relate to your story.
2) give attention to pacing
It is very important to make peace with the pace of the story because if the speed of the story will not cooperate along with the imagination of The reader then the story will become a hard read which will automatically decrease the demand of the book in the market.