H/t to Instapundit to point out the John Scalzi question on just what is and is not a science fiction based movie. As I come from the SF Lit side, I am biased and tend to put a more rigorous basis on SF, but that basis also captures movies that no one would classify as SF! In the SF media community you will get different answers, but for me there are some touchstones and hallmarks of SF that must exist, regardless of presentation media, for it to be SF.
First off it must be within the known or extrapolated known realms of science. Thus Angels flitting around on wings because 'God made them to do that' doesn't cut it as SF while genetically engineered pterosaurs from Jurassic Park would. Time travel stories fall either into fantasy (supernatural power) such as Somewhere in Time or SF (machine using some extrapolated known laws to function) like Back to the Future. If you are The Man Who Could Work Miracles you are in a fantasy setting and if you are stuck in Things to Come you are in a fascistic SF setting.
Now notice that this picks up some wide number of films not normally considered SF, but which are rigorous extrapolations from known technology, like The Guns of Navarone. By positing Nazi Germany putting together its large cannon technology (which it had) plus its primitive radar (which it had) you now get radar guided large guns used for anti-shipping purposes and putting a lot of anti-aircraft equipment on top of that, makes it pretty well defended. Even written after the fact, it is a prime piece of speculative SF and a definite sub-genre known as 'Alternate History' (Alt History). Minor sub-plots during real engagements are fictionalizations of those engagements, not SF. Utilizing the known in novel methods and rigorously going through them: that is SF.
Second is the rigorous use of technology to show its social and societal fallouts and then going through those in a rigorous fashion. This is a major stumbling block, as you can have something utterly fantastical that isn't explained or easy to extrapolate and carry it out in a rigorous way to come up with a great story! You can postulate some derived alien capability to do something and not explain it, but cover its outcome in a thorough manner and it LOOKS like SF: it has all the settings and trappings of SF, and just shoves the explaining and extrapolating into never, never land. Alien capability dropping in from space doesn't make it instantly SF, however, as the basis does need some backing to it, otherwise you are in a fantastical realm of angels, demons and just adding 'aliens' to the mix. As an example of this, there is the Invasion of the Body Snatchers (1978), which I count as fantastical fiction, but not SF, and the The Thing (1982) which *is* SF as it explains how The Thing operates. Both do excellent jobs of extrapolating from this outside influence that arrives on planet Earth, but only one really takes it apart in a semi-believable way. Also both are remakes, a Hollywood genre in, and of, itself. Just because you have spaceships and aliens, doesn't mean that you have SF (or in the case of IBS space seeds drifting through the cosmos... so much for gravity, huh?).
As Mr. Scalzi ventures into the comic book superhero business, lets take a look at a few of those to see where they fit so far. Spiderman is a great character study, but post gen-engineering a human body to let you wind up with something nice is currently outside the realm of possibility and by pure chance it goes out to fantastical... just ask poor Seth Brundle in The Fly which, no matter how weird it is, is actually closer to SF as it attempts to put a decent bit of reasoning down for it. Iron Man falls down on power source (damned he should have used beamed power), and have you actually done the estimates of how much it takes to make a human body move that fast in flight? It has all the rigorousness and techno trappings you could ask for, but if they wanted a power source they should have taken it one more step... it would have killed the story line until they figured out how to do it, but thems the breaks in making SF. Taking The Dark Knight, we find actual uses of real technology that only require engineering, not fantastical discovery. I'm writing a bit of Batman fiction for personal fun right now, and it is damned fun to do and you don't even have to do post-known extrapolation to do it in the past, just rigorously apply what was known to come up with something workable.
For sociology and demographics, you can have all sorts of neat techno stuff and not be SF. Logan's Run falls into that as it was not a valid nor rigorous extrapolation of demographics, even as a novel. Blade Runner, by utilizing technology ina way to support known trends of industry and population density is SF, and a great flick once you get rid of the narrator... Alien is SF, following trends of technology and industry and postulating and organism that has adapted to cross-species parasitism, and Alien 3 is religious metaphor, not SF, even while using the successful Alien concept as a platform. Did *that* series lose its way, or what?
Just how much and how quickly does society change and why? If you have changes in understandings of science and creations of new technological tools, then just how quickly can humanity adapt to them? We went from expensive cell phones in the 1980's to ubiquity of them in two decades or less. A 1980's 'luggable' computer is now thoroughly captured in an iPhone, and as a result we are more tuned in to ourselves as individuals and create less common society around us with our fellow man. By creating common and cheap connectivity, we dissolve social bonds on a local level and allow such things as radical Islam to spread world-wide as the dominant social setting that breeds it is no longer cut off by distance. These are *minor* changes in technology, and yet they have caused deep and abiding societal drift and anomie in almost no time at all. Larry Niven's early Known Space works in the realm of Lit. SF did a great job of presaging this, but on the side of the rest of the media...
Things to Come, mentioned above, no matter how horrific its idealization of a fascistic future is, does a good if not excellent job at destroying global society to allow this global takeover, while Logan's Run, doesn't. Death Race 2000 along with something like Rollerball, both dystopian fantasies, really did picture a steep decline in human society that you really did have problems seeing, while The Running Man was humorously chilling in that you could picture it, all too readily. Soylent Green fails as, lets face it, humans are a bit too calorie intensive to be a good food source for a world both over-run with humans and without enough to eat... think about that for a moment, it is a self-solving problem: not enough environment and population plummets, not goes up. And if you can make space ships to go through the solar system, like in Silent Running, then how hard would it be to make a few space stations to house forests? Artistically these may be good films, but SF-wise? If you have a horrific ecology and industrial societies running out of material you get the world of Mad Max/Road Warrior, both of which carry through on the societal logic to its end points. All of these would normally get put into the SF category by most reviewers, but to me you wind up with half of them (MM/RW being one world).
When it comes to the ultimate place of man's society in films, the rule is that dystopian films rule the roost as no one wants to go to a film in which mankind actually works out its problems in a non-repressive way. Often its not even mankind, itself, that causes the repression, but our works. This line of thought started very, very early on with Fritz Lang's Metropolis which is pretty much where the compelling story line of robots putting the future of humanity at risk is born. It was SF for its day, taking the known concepts of class warfare, automation and man's role in this process we call 'work' and the introduction of machines that move man away from work will have its own allure, beyond that of the actual beauty of the robot, itself. Doing that extrapolation of machine efficiency to human labor, casting it against social theory of class warfare and Metropolis is the result.
Compelling cinema can be had from this set-up, with Planet of the Apes doing just that in its use of the speculated about suspended animation for humans to cross light years at relatively slow speeds. Humorously, the equipment created to actually track the ship does NOT tell everyone that it had a minor malfunction so that they really didn't go much of anyplace. It is a minor flaw (a nit picking one, to be sure), but this ship of such technological genius, such heroic discovery, can't even tell where it is? The constellations and stars in them would not have drifted that much and it should have gotten the general neighborhood being in or right close to the Solar System *right*. It is used as a plot device, and yet, with just a bit of work, it could have been avoided. In general, if you are putting a film together, you want to pack the absolute worst inconsistent parts into the front end of the film: they are either forgotten by following content, or they are the 'taste' of the major plot to see where you wind up with such things happening. Still it is a technical flaw, not a religious belief that mankind will over-run the planet with his own progeny. There is much in the way of SF that has such problems and is still SF because the underlying extrapolation is sound, not fanciful. Plus two thousand years is just about the right time span to achieve that complete overturning of the order in human society.
Poking fun at dystopian futures is also a good staple of the film world, though there are far fewer of those. If you see Metropolis then the counter point to that is Chaplin's Modern Times, which also fares excellently because he was trying to show where that machine age of his ends up. One of the major bits of presaging he does is the man who has many careers in his life! Eerily that has come true, in spades, and following Chaplin from working class, production line stiff to jail to supposed Communist organizer to informant to being in the '30s bread lines to... how many jobs does he have in this film, anyway? Singing waiter, anyone? Because he tries out for that, too. In looking at his own Modern Times, we see a different, less class warfare form of view taken, and it is riveting comedy, even when it is compelling SF as an examination of how social orders do change and how change shifts opportunities and lives.
If you take advanced technology, somehow isolate it to the idle rich and don't let one, single bit of information that you have such advanced technology available and decide to use it in nefarious and, ultimately, chillingly illegal ways you get The Stepford Wives. Such highly advanced robots and NO ONE knows about them? Really? Even back then, it was looking like robots of this type would require a hugely advanced industrial base to make them... reading Asimov from that era points to that. If, like in I, Robot, you saw tons of robots around all the time, you might be someplace. And pretty much have sexbots available, like in AI. Really, if the men needed those kicks and could afford the sexbots, they would be readily available and those marrying such men would know that, at the start. Postulating the technology without the backing of the larger industrial base is a non-starter. You can make a great thriller out of that, but that doesn't make it SF, as technology is just a stand-in for previous generations of demons and devils and otherworldly super-beings. That was also done in Demon Seed, but just where are all the others of the Proteus line (numbers I, II and III) that would make anyone *want* to make a Proteus IV?
When you have machines malignant to humanity and add in an automated industrial set-up you get The Terminator, which is far more rigorous in that extrapolation than The Stepford Wives. Like Planet of the Apes, The Terminator places its 'no good way to explain it' time travel at the front of the film. More deeply it is an update to Colossus: The Forbin Project, which itself had some techno-gaffes on computer sizing and ability but was the, then, reasonable extrapolation of such. Skynet from The Terminator starts out as a Colossus-like machine, and in Terminator 2: Judgment Day that is updated to being a coordinator of a computer virus (pan-platform, global) that allows Skynet to jump from coordinator to controller in a matter of minutes. The first Terminator film is one of the most compelling Action/Adventure flicks and Dystopian future films around, for all that we only see the effects of it, those effects are horrific. Leaving aside the question of time travel, it is SF in a pure sense. Leaving aside time travel in the second film also makes it a hugely compelling film, in many ways superior to the first. Of course if you leave aside time travel in both films, you don't realize that past history is changing before your eyes (which makes for a hugely compelling idea about time travel, in general, that is never discussed). Where The Stepford Wives falls down is on industrial production and capability, and that added in makes for a wholly different, far more compelling future vision outside of a single story like the first two Terminator films... with the third falling far short due to lack of coherence and continuity: how does Skynet get such great machines when, in the first film, it is being besieged and brought down in its central computing area? Thus the problem is the part you leave aside: time travel. Again the series goes downhill quickly without the continuity.
From this conception, for me at least, SF does need to be rigorous, have founding in what is known and what is reasonable to extrapolate from that. Throwing in technology to make a plot work is just that: throwing in technology because you can't think of the underpinning of it to make a compelling film and want a 'MacGuffin'. Actually, Alfred Hitchcock did a splendid job with his use of such things as shown in Notorious, and its plot device turns out to be something that would be a real mover and for more than just a thriller, and Hitchcock claims he had been followed by the FBI for using it! True SF is that which would cause such notice.
He would also use social commentary to view how people within society react to serial killers with sexual and other overtones. While hearkening back to Jack the Ripper, The Lodger: A story of the London Fog is a deep investigation into the psyche behind mass-media driven hysteria and one of the most compelling views on how media impacts culture that has ever appeared in film. You would think that a film that had NO high-tech and is just doing social commentary would not be SF: but by carrying through how past hysteria has been built, the result is that of a rigorous extrapolation for such future events. We must remember this is the era (1927) when ONLY Jack the Ripper had been of note: no Boston Strangler, Son of Sam, Green River Killer, John Wayne Gacy or Charles Manson. That list is, itself, a chilling indication of how Hitchcock's examination and extrapolation had compelling legitimacy for all that it was a fictional account. Without needing technology, he performed a feat of compelling SF and didn't even mean to do so.
Hitchcock's examination of 'war by other means' in Sabotage, goes beyond the pre-WWII setting and presages the era of terrorism in the modern world. By divorcing those doing the dirty work from any Nation, any ideology, and yet making it clear they are under control of some Nation or group, the chilling horror of how individuals who seem so nice can be deadly killers is given front stage. This, too, is extrapolation from the known, but by doing artistic work to ensure that the those doing such work are 'timeless', Hitchcock does a timeless analysis of how and why such seemingly pleasant individuals can seek to hide such horror before doing it.
As I pointed out in the beginning: SF is not limited to one type of film and by using a thorough-going analysis and presentation of that, you can achieve good extrapolation based on known science and behavior. That does not make such works primarily SF, as we are too captivated by the technological 'eye candy' to understand that technology is a gloss over the social structure and individuals that comprise it. Being a 'saboteur' without apparent connection to the 'Great Game' between Nations makes that individual unlike others and willing to toss aside civilized means when it suits them to do so. That extrapolation is based on how anarchists and communists acted previously, and by generalizing it, Hitchcock turns it into a scientific rendition of the human condition done on film as a thriller. By using the known and going into the unknown with a good use of continuity and extrapolation, you get SF. Deadly machines don't require there to be Terminators, and using radar with accurate cannon technology yields astounding results... yet one is SF and the other isn't because of its WWII setting. All that is bright and flashy with technology is not SF, and all that is, apparently, mundane is not that when handled properly by a writer, director or producer just willing to take that one, extra step and ask: 'What if...?'