They usually balance the two genres well. A good book like 'The Host' by Stephenie Meyer (which has elements of both sci - fi and romance) doesn't let one overshadow the other. The romance is believable within the science - fiction setting. The settings are often unique, whether it's a post - apocalyptic Earth or a far - flung alien planet. This uniqueness makes the relationship between the characters stand out more. Also, the plot should be engaging, with twists and turns that keep both the science - fiction and romance aspects interesting.
One characteristic is that they introduce new ideas. In 'Ancillary Justice' by Ann Leckie, the unique concept of a ship's AI being split into multiple bodies affects the main character's relationships. Good science fiction romance books also have emotional depth. The characters' feelings for each other are developed gradually and realistically. It's not just about instant love. And the conflict in the story is not only related to the relationship but also to the science - fiction elements, like in 'Illuminae' by Amie Kaufman and Jay Kristoff, where the characters have to deal with a deadly virus and a war while also falling in love.
Good science fiction romance books often have a well - crafted world. For example, in 'The Long Way to a Small, Angry Planet' by Becky Chambers, the world - building is detailed, with different alien species and their cultures. This provides a rich backdrop for the romance to develop. The characters are usually complex, with their own goals and fears. In science fiction romances, the science - fiction elements, like advanced technology or space exploration, add an extra layer of excitement and often pose challenges to the relationship. For instance, in 'Dark Matter' by Blake Crouch, the concept of parallel universes affects the relationship between the main characters.