(giggle) I always end up late to the party…
Let me start by correcting some false assumptions about my assertions regarding science fiction. I could do so by responding point-by-point to the statements made in my absence, but I think simply expounding my philosophy on the subject would do a better job (and be better in keeping with the idea coosa had in his OP) 
For those interested, the thread under discussion can be found here:Best Science Fiction/Fantasy It’s lengthy and it’s from MPSIMS, so be advised.
Science Fiction is a specific type of writing. It often involves elements of fantasy, but it consists of a very basic writing premise, which, if absent, means that it isn’t science fiction. That premise is an assumption by the author of an advance/breakthrough/discovery of scientific nature (including fuzzy sciences) which has an affect on society, explored through the millieu chosen by the author. For example: In the classic science fiction story by Robert Heinlein, The Roads Must Roll, he postulates a future (well, it was HIS future at the time; IIRC the events transpire in something like the 1960’s) where society in America developes huge moving conveyor belts that act as freeways, on which people move without having to use automobiles, and on which restaurants and businesses can be carried. He then examines the social impact of a class of specialized employee (road technicians) who have the power to wreck economic havok by shutting down the roads. You can read this story in his collection The Past Through Tomorrow.
Let me give some examples of what are NOT science fiction under this definition:
Honor Harrington - These entertaining books are simply Horatio Hornblower set in space. The fact there are spaceships has nothing to do with the plots; indeed, the author, David Weber, does little to differentiate the challenges of a culture surviving on seperate worlds from the difficulties faced by England and others in running an empire seperated by large oceans.
Princess of Mars - And all the other Burroughs novels of John Carter and or his Venusian counterpart, Carson of Venus. These take place on other planets in the solar system, but they are totally fantasy; there is no discovery or advance that results in being on Mars.
Now, having described science fiction, we can begin to explore what makes for good science fiction, and what can go wrong with science fiction.
Good science fiction has certain elements: obviously, a good writing style; an interesting plot; a premise someone is willing to read about; a lack of incorrect scientific facts. I highlight this last part because it is over this issue that coosa and I disagreed.
Obviously, to be science fiction, there has to be something about the story that is not currently possible. On the other hand, the story shouldn’t include things we know are not true or cannot be true. As an example: it was reasonably okay for authors in the 30’s and 40’s to speculate about life on Venus and Mars; it would be horribly stupid to write about a civilization on Mars that left the ‘canals’ now. We know there are no such things: they were simply the result of low resolution photography from the early 1900s. So when you read something like Heinlein’s Space Cadet, it isn’t the fact that he has spaceships running around the human civilization of the inner solar system that makes you cringe; it’s the description of Venus as some sort of swamp under the clouds and the existence of amphibious Venusians that has you rolling your eyes and wondering what sort of idiot wrote the book (mind you, you probably thought something similar the first time you read the book after getting older than 15, but that’s another story).
This type of mistake should be differentiated from making an assumption of a future advance that seems to us unlikely. For instance, we currently cannot travel faster than light, nor does it appear possible that we shall ever do so. This by itself does NOT make fiction about interstellar travel stupid or bad. To assert that we cannot ever discover or invent a way around this limitation would be similar to a Roman citizen discounting the story of Icarus on the basis that man can’t fly. But, if you make an assumption that is too outlandish, it may make it hard for the reading public to swallow the premise. This is the reason good science fiction writers have stopped talking about spaceships that simply go faster than light; they usually substitute some sort of space folding drive or tunnelling drive, or they work with such things as ramscoops, light-sails, etc. and accept the fact that a trip to Alpha Centauri would take a LONG time.
Which brings me to what I said about the Anne McCaffrey Dragonrider novels.
First off, one has to keep in mind that the first of these books, Dragonflight, started out as a novella called Weyr Search that won the Hugo and Nebula awards in 1968 for best novella. The novella includes the material at the start of Dragonflight where the weyr riders (including F’lar) are on search, and land at the hold where Lessa is scheming revenge on the Lord, who usurped the position from her family and slaughtered all her relatives. Now, no one would call this novella science fiction; it was classic fantasy (dragons, castles, ESP, swords (or, at least, knives), a very medieval setting. It was only when it became a full length novel that Ms. McCaffrey mutated it into science fiction. How did she do that? She introduced the following idea: that the planet Pern was periodically subjected to rains of ‘thread’ which coincided with ‘passes’ of the ‘red star’ every 200 years or so, against which the dragons and their riders were a bulwark. Now all that alone is still fantasy, but Ms. McCaffrey wouldn’t rest there: she insisted on having her characters try to solve the mystery of the ‘thread’ and in so doing, attempted a scientific explanation for the whole thing. As originally postulated by Ms. McCaffrey in the first two or three of the books, the ‘thread’ consisted of mychorrizoid spores which emanated from the ‘red star’, a planet with a highly eliptical orbit that reached perihelion every 200 years. When the planet got close to the sun, the surface heated up, causing the spores to become active and attempt to travel to Pern, where they were a dangerous form of life that killed off native Pernian life. Into this situation Earth colonists had landed some years previously, then been forced to abandon their modern Earth culture and turned into the medieval culture depicted, but only after the original colonists had managed to begin a breeding program that, among other things, created the dragons. (whew!)
First off, let me say that I enjoyed reading most of Ms. McCaffrey’s works; I only stopped buying them when she developed that terrible disease of successful SF/F writers whereby instead of one book every year or so they end up publishing two or three every year, making it difficult to keep up with them. She used to write entertaining dialogue, interesting plots and fun stories.
But let’s examine the science fiction of all this:
Premise: a planet that reaches perihelion every 200 years resulting in a rain of spores on the neighboring planets.
We will dispense with the somewhat technical difficulty that occurs with this premise from a gravitational mechanics standpoint (frankly, none of the orbits would be stable and Pern woulda been in cinders long ago). What we have are two obvious errors that Ms. McCaffrey created in the premise: why wouldn’t thread fall at night (when it can’t be seen and, thus, can’t be fought in the air) and why does thread fall continuously for 50 years during the ‘pass’ of the star even when the sun interposes itself between the ‘red star’ planet