Chip Delany, in just two paragraphs, nails to the wall the very best and worst aspects of science fiction:
You can put together more interesting combinations of words in science fiction than you can in any other kind of writing—and they actually mean something. You can say things like, “The door dilated” (as Heinlein did in Beyond This Horizon), and it implies both design and technology, as well as whole sets of social and experiential differences. When you say things like, “Her world exploded,” you are not just giving a muzzy metaphor for a female character’s mental state. You reserve the margin for the words to mean that a planet belonging to a woman blew up. Thus SF is sensually pleasing to work with, simply at the level of language.
I think what happens with mundane or naturalist fiction is that these characters succeed or fail in what they try to do, but they succeed or fail against the background of the real world so that their successes are always some form of adjusting to the real world. Their failures are always a matter of being defeated by the real world. So in a funny way the only thing that mundane fiction can talk about is either madness or slavery—those people who adjust to the world and therefore are slaves to it, or those people who are defeated by the world and are therefore mad because they shouldn’t have tried in the first place. But I do think there’s something else: in science fiction, because success or failure is measured against a fictive world that is itself in dialogue with the real or given world, that dialogue is much more complicated, richer, than in realist fiction. Of course, we all know that there are many things about science fiction that are predictable. One of the problems—now that I’ve given this account of SF’s potential—is that, for a long time, SF was a kind of marginal writing, with many hard and fast conventions. You can do absolutely anything in it. But when you can do absolutely anything, you tend to fall back on the conventions.