I have a half-baked theory about genre fiction--that, on the whole, it serves a propagandistic purpose. Crime novels are about social problems, romances are about wish fulfillment, westerns are about politics. Science fiction has its roots, I think, in our ambivalence about science, and the ways it can be used as a force for good or evil; it's always been forward-looking, has always tried to tackle problems that don't yet exist.
As such, its aims have always been admirable (the right-wing screeds of Michael Crichton notwithstanding). But often, the things that make books great are missing in science fiction. Characters too often stand for things other than themselves, and so the exploration of consciousness--of what it means to be human--gets short shrift.
A few writers have tried to go against the grain, though. Octavia Butler springs to mind; so does Ursula LeGuin, whose The Left Hand of Darkness reinvented gender in order to probe human nature in new ways. And Kim Stanley Robinson's Mars Trilogy explored, among other things, what living for 200 years would do to a person's mind.
Lately though I've really been into China Mieville, whose trilogy of novels set in his invented world of Bas Lag somehow manage to bundle all that is good about SF with almost everything that's good about literary fiction--his characters think deeply, in a deeply strange place. You could accuse him of overwriting (I do anyway), but the exhaustiveness of his imagination almost requires it. There's just so damned much to say. The best of these books is probably The Scar, which is about an embittered linguist on a transcontinental sea voyage that is interrupted by pirates, who steal the ship she's on and add it to their thousand-year-old floating continent of booty. The story takes place in the equivalent of earth's mid-ninteenth century, but instead of one sentient species, there are dozens. There is science, but there is also magic. There is oil to be drilled for--but there's also something called "rockmilk." There is slavery, but the slaves are, more often than not, genetically-engineered mutants or man-machine hybrids. And all this makes perfect sense, and does not seem remotely silly.
I wish more people with Mieville's chops would apply it to science fiction--I think there's a lot more to be done there than many writers have bothered to do. I've been on my own case for years, actually, to write a science fiction novel, and one of these days I'll do it. But I tried it once, and man, it was really freaking hard. I dropped it after forty pages and did something else.
Maybe what it takes is to be superhuman--maybe you've got to be science fiction to write it.