The first is that Science fiction is the literature of Change . The world of the science fiction story is fundamentally different from the world of the reader. While being different, logic and reason still apply, and every difference has a rational cause. The change could be an extrapolation of current science or technology, or it can be a new paradigm, but the story must explore the consequences of change in a rational way. Some science fiction does not fit in that definition, though.
Some science fiction treats the science as the underlying metaphor.
Schwarzchild Radius is set in the trenches of the Russian front of World War I with Karl Schwarzchild. The trench is the Event Horizon. Every event in the story has a parallel with General Relativity. Relativity is also the central metaphor in Einstein's Dream . While this book was not marketed as science fiction, good science fiction has a broader appeal than most people realize.