It strikes me that one of the many reasons that science fiction is not, as a genre, booming with good and original works that are of memorable excellence at the moment might be that our assumptions about people have matured.
The wonderful and caustic Thomas Disch writes interestingly about the immaturity of science fiction-- not only audiences, but themes. Well, I know I read more of it when I was a teen, although arguably that was because I still had all the great classics before me... and I know there is a lot of SF that I find laughable, even endearingly embarrassing, now. Like a high school photo. It's true: late Heinlein's post-sexual-revolution fantasias, Dick's paranoid Freudianism, and even Herbert's grotesque wish-fulfillment rise-to-power narratives are juvenile in their essentials.
Any of the glorious war-vs.-the-aliens books, well, glorify war... you will note that our volunteer military pitches its glorifications to youthful potential recruits. Only in SF, the sinister alienness, the us-or-them dimensions of a war, are far more incontrovertible and indisputable than in the messy single planet upon which we find ourselves, where our enemies are potentially brothers. It is a clean universe of glory in a Manichean opposition of light vs. dark (sometimes literally... yes, I'm looking at you, Star Wars.)
It strikes me that the over-and-doneness I ascribe to the nature vs. culture debate in the preceding post (where I followed that up with a long rant about culture... I suppose I should have clarified first) may affect how we create and receive sf.
If nature is anthropogenic, which is a (much more current) viewpoint I will support and defend, prying apart nature and culture is impossible. And that makes it difficult to have absolutes, particularly absolute human or inhuman nature, without involving religion.* Absolute evil must be mystic, not biological; aliens become sympathetic; our nice neat Manichean black and white goes away and we are left with messy, depressing warfare.
Is the slow, ongoing fall of sf a result of battle fatigue, shame and guilt, and a feeling that instead of a hundred page book of how to whoop 'em, we should be reading a six hundred page history of negotiations? Could it be that there IS a sphere of resistance to the sound-byting that has fragmented and polarized our nightly news?
Dunno. I got up early, what can I say?
(*I'm going to say that if you write a story about an alien who is raised by loving humans in every kindness, he will be -- MUST BE -- human; I don't think I can say the same of a story about a hand-raised demon. See where I'm going with "religion" here?)