Aaron2 said:
Boring and/or preachy --> Science Fiction.
Fun and/or entertaining --> Science Fantasy.
Aaron
Challenging and Stimulating --> Science Fiction
Silly pseudo-mystical Fluff --> "Science" Fantasy
In all seriousness, I really agree with China Mieville's sentiments- there is no difference between fantasy and science fiction. The simplistic definition is that one is plausible and the other is not falls somewhat flat. Even if a novel or story conforms with what is known by modern science, it is limited to those areas of modern science of which the author has a grasp. "Science" does not exist as a monolithic entity- it is a massive collection of work, all to often conflicting, done by millions of researchers around the world- often completely independent of one another. All one has to do is listen to modern debates over cosmology, string theory, chaos theory or the mind/body problem to know that there is no universally agreed upon "scientific establishment". In addition, what is "science fiction" that is based upon information that has now been largely thrown out by the court of opinion? One only need look at much sci-fi written forty years ago to see how quickly such material becomes "fantasy".
For instance, I've often heard Star Trek described as science-fiction, while Star Wars is described as science fantasy- but wait! Star Trek includes beings with supernatural and unexplained powers, psychics, plenty of voodoo physics, etc... all provinces of fantasy. Many extremely good authors of what is known as "hard science fiction" almost verge on this realm at times, too- one must only look at Stephenson's magical and theological musings in
Snow Crash or Vinge's pseudo-magical cosmology in
A Fire Upon the Deep to realize that these guys aren't necessarily much harder-nosed than the creators of
Final Fantasy VII or
Perdido Street Station.
Really, all speculative fiction is that- fiction, regardless of whether you add the moniker "science" or "fantasy" to it. I prefer the term "speculative" to describe it all, as it posits that it is fiction about a hypothetical reality, rather than one based in (presumed) history or (presumed) contemporary society.