In short, the straw of a manufactured realism with which the sf writer makes his particular literary bricks must be entirely convincing to the reader in its own right, or the whole story will lose its power to convince.- Gordon R. Dickson
The future depicted in a good SF story ought to be in fact possible, or at least plausible.- Frederik Pohl
A lot of the definitions of science fiction that I've read tend to deal with what makes for good science fiction - not a bad thing to have definitions that concentrate on quality, although I think that some of the motivation for that sort of definition comes from a kind of defensiveness about the genre, as per Sturgeon's Law.
For a perfect example of the line that divides good SF from bad SF, let us turn to the popular media - okay, the BBC, but still - and take a look at the reborn Doctor Who.
In the first season, with the excellent Christopher Eccleston capably filling the Doctor's shoes, there was a two-part episode: The Empty Child and The Doctor Dances. Set in WWII London, the Doctor is presented with the odd phenomenon of an inhumanly powerful child, wandering the streets in a gas mask while looking for his mother. Even odder, the child's condition appears to be contagious - people are mysteriously changing into gas-mask faced entities requesting their mummy.
This is explained as being the work of military medical nanobots from a crashed spaceship, nanobots that have no idea that the model they are using to rebuild the population of London is in fact a flawed one (people not normally having gas masks for faces, even in London) and the boy's powers being the result of being brought up to alien military spec.
This would qualify as "good" SF by my standards: an apparently inexplicable situation which is logically explained within the context established by the plot.
Sadly, David Tennant, Mr. Eccleston's equally gifted replacement, is not always as ably supported by plotline. In The Idiot's Lantern, once again people are falling prey to a mysterious ailment, this time in 1953 London: after watching television, their faces vanish. (I've often thought that might happen.)
After seeing a sort of corral full of faceless victims, the Doctor finds a room full of TVs, each one containing a missing face. Apparently this is because an alien entity known as the Wire is stealing energy from people.
What? Why in the world would that cause their faces to vanish? How do these faceless, mouthless, noseless remnants breathe? Shouldn't they all be dead in about four minutes? Sadly, this episode rings the bell for bad SF, where the science fiction element is really just for show, intended to create an interesting visual effect but with nothing in the plot to explain why and how such a thing would happen.
And, as a postscript to the whole thing, it really does make me wonder if I'd feel safe living in London.
