I've always taken it as a given that science fiction requires at least one central science-fictional conceit. In other words, if you remove the sf furniture, and your story does not change... well, then it's not science fiction. It's "skiffy". And skiffy is bad.
I don't remember where I picked this up from, although I think it's common parlance in British sf fandom. Wikipedia is no help - it describes "skiffy" as a "deliberate humorous misspelling or mispronunciation of the controversial term 'sci-fi'". No mention of skiffy as a description of a story (or its shortcomings). The Turkey City Lexicon, however, calls it the "Just-Like Fallacy" - a "SF story which thinly adapts the trappings of a standard pulp adventure setting".
To my mind, Wilson Tucker's original definition of space opera as "the hacky, grinding, stinking, outworn space-ship yarn" was in part a dig at the sort of stories I know as skiffy. Certainly, Galaxy magazine's later pendant to the term - a series of ads featuring Bat Durston, under the banner "You won't find it in this Galaxy" - were pretty clear as to what they thought was bad sf. Their example was a western in space - a well-known and much-derided form of bad science fiction.
However, a recent discussion on a forum has made me question this given.
I still think it holds true - I can't see the point in making a story science fiction if all you're doing is slapping a thin coat of rocketships-&-rayguns paint on it. But how relevant is an insistence on a science-fictional conceit in a post-Star Wars genre? That film was little more than a hodge-podge of story archetypes dragged by the scruff of their necks into a space opera setting.
David Weber has done something similar with his Honor Harrington series. His heroine is Nelson in Space - even down to losing an eye and having an adulterous affair. The People's Republic of Haven is Revolutionary France - the chief villain is even called Rob S Pierre! Where's the central science-fictional conceit in the Honor Harrington series? What is it about the series' story-arc that means it can only take place in the Honorverse? There are plenty of science-fictional ideas in the books, from Weber's take on faster-than-light travel, the weaponry used by the various warships, the... er, well, the furniture, basically. The Honor Harrington novels are very successful. Yet they aren't that much different from Bat Durston. True, a female admiral could never have existed during the Napoleonic Wars, but turn Honor Harrington into Horatio Harrington, the Warshawski sail into a canvas sail... and you have essentially the same story.
So... is the genre nothing more than its furniture? Is that all sf readers really need for a story to meet their definition of science fiction?