The trouble with mimetic fiction isn’t that you can tell what’s going to happen (I defy anyone to guess what’s going to happen in Middlemarch, even from half way through) but that you can tell what’s not going to happen. There isn’t going to be an evil wizard. The world isn’t going to be destroyed in Cultural Fugue and leave the protagonist as the only survivor. There aren’t going to be any people who happen to have one mind shared between five bodies. There are unlikely to be shape-changers. In science fiction you can have any kind of story—a romance or a mystery or a reflection of human nature, or anything at all. But as well as that, you have infinite possibility. You can tell different stories about human nature when you can compare it to android nature, or alien nature. You can examine it in different ways when you can write about people living for two hundred years, or being relativistically separated, or under a curse. You have more colours for your palette, more lights to illuminate your scene.
Now the problem with genre fiction is often that writers take those extra lights and colours and splash them around as if the fact that the result is shiny is sufficient, which it unfortunately isn’t. So the most common failing of genre fiction is that you get shallow stories with feeble characters redeemed only by the machinations of evil wizards or the fascinating spaceship economy or whatever. What I want is stories as well written and characterised as Middlemarch, but with more options for what can happen. That’s what I always hope for, and that’s what I get from the best of SF.
–Jo Walton, “What a pity she couldn’t have single-handedly invented science fiction! George Eliot’s Middlemarch,” republished in What Makes This Book So Great