Some interesting ideas here from the incredible Lois McMaster Bujold.
Writer Guest of Honor Speech, Denvention 3 by Lois Bujold (Lois McMaster Bujold) on Myspace
Romance and SF seemed to occupy two different focal planes, to steal another metaphor, this time from photography. For any plot to stay central, nothing else in the book can be allowed to be more important. So romance books carefully control the scope of any attending plot, so as not to overshadow its central concern, that of building a relationship between the key couple, one that will stand the test of time and be, in whatever sense, fruitful. This also explains some SF's addiction to various end-of-the-world plots, for surely nothing could be more important than that, which conveniently allows the book to dismiss all other possible concerns, social, personal, or other. (Nice card trick, that, but now I've seen it slipped up the sleeve I don't think it'll work on me anymore.)
In fact, if romances are fantasies of love, and mysteries are fantasies of justice, I would now describe much SF as fantasies of political agency. All three genres also may embody themes of personal psychological empowerment, of course, though often very different in the details, as contrasted by the way the heroines "win" in romances, the way detectives "win" in mysteries, and the way, say, young male characters "win" in adventure tales. But now that I've noticed the politics in SF, they seem to be everywhere, like packs of little yapping dogs trying to savage your ankles. Not universally, thank heavens -- there are wonderful lyrical books such as The Last Unicorn or other idiosyncratic tales that escape the trend. But certainly in the majority of books, to give the characters significance in the readers' eyes means to give them political actions, with "military" read here as a sub-set of political.