Ah, the question that burns like a fire in the soul of every science fiction fan. Finally we shall know the answer.
To answer this question I only need to focus on those things that are unique to science fiction movies so there will be little talk of plot and characterization. This is gonna' get a little technical so I'll start out with a few relevant concepts.
A science fiction movie contains tools or artifacts which current technology cannot duplicate. Further more these technologies are integral to the plot or setting of the story such that if removed the story would no longer logically hold together.
When the science and technology in the movie/story extrapolates form but does not violate known scientific principles this is known as Hard Science Fiction
Science Fiction Convention - Some imaginary technologies are so prevalent in science fiction that little or no explanation as to how they work is necessary for fans of the genre to suspend their disbelief, I'll list a few here. Time machines, faster than light travel, energy weapons, hover craft, and aliens. Each of these requires only the barest explanation, audiences just expect them and except them in their science fiction.
Fantasy Stories – Stories in which real scientific principle are violated and no imaginary technology is needed to do so. Characters merely have 'special' abilities, or have special knowledge which allows them to bend nature to their will.
Science Fantasy - Science Fantasy is a fantasy story in a futuristic setting. Also known as bad science fiction.
What isn't science fiction?
First one more definition. Unconventional Science Fiction Technologies – Technologies which require no machinery in order to work. Of course they are allowed but you must explain them in order for them to qualify as science fiction.. Flying, time travel, or teleportation without machines. Immortality, telekinesis, or any other sort of psychic power.
Movies in which popular science fiction conventions are not followed and there is no consistency in the effects caused by the application of the technologies in the movie are not science fiction. They are science fantasy.
Now on with the show.
What makes a science fiction movie bad?
A movie can be considered bad science fiction if the plot of the movie depends on the violation of any of the following principles.
The science in the movie contradicts itself. The science in the movie contradicts known scientific principles without explanation and doesn't follow popular science fiction conventions.
The science in the movie does not follow accepted science fiction conventions without explaining why.
The characters in the movie are unable to draw obvious conclusions based on the scientific knowledge available to them.
Can a bad science fiction movies be entertaining? Absolutely, if the movie makers are good enough salesmen but that doesn't make them good science fiction. Star Wars anybody.
What's the difference between bad science fiction and fantasy? One might imagine the events in a science fiction movie could actually take place in the future given the advancement of technology. In other words science fiction movies have the 'pretext of plausibility'. Fantasy movies do not, they are strictly fairy tales. Therefore bad science fiction movies are not fantasy.
A few examples of bad science fiction movies.
Red Planet - Violated Rule number three. The astronauts didn't know they could breath the Martian atmosphere even though they were scientists there to study the planet.
I Robot – This one really makes no sense. The most respected robotisist in the world is being held against his will in a robotics laboratory. So he builds a super advanced robot but instead of sending the robot to the police for help, decides to talk the robot into killing him in order to draw attention to the fact that he is being held against his will. The killer robot then runs away and hides. An investigation into the murder subsequently reveals the main plot about a computer trying to take over the world. At the end of the movie they let the killer robot go because there is no law against robots killing people. This one breaks all the Rules and is unwatchable by science fiction fans.
Armageddon – A killer asteroid is headed for earth. So astronauts use a nuclear bomb to blow it apart. Of course this should have created a lot of smaller but still deadly asteroids. Rule number three violated. It was a fun movie anyway.
What makes for a good science fiction movie?
Good science fiction movies follow these principles.
The technology in the movie extrapolates from but does not violate known scientific principles. This of course is hard science fiction.
When known scientific principles are violated the movie explains why and how.
When known scientific principles are violated without explanation the movie follows popular science fiction conventions.
Now a few good science fiction movies.
I won't say to much about these movies in case you haven't seen them. You really should see them they are superb.
Blade Runner – This movie is filled with compelling characters each one trying to discover what it means to be human. Even if they are not.
Planet of the Apes – This movie very effectively tackles racial discrimination. No not the remake, the original movie.
Forbidden Planet – This movie tackle the question of whether or not some technology is to powerful to wield. This one is dated so give it some slack.
Serenity – Originally a TV show called Firefly. Serenity is a really fun film with great characters. All though it doesn't effectively deal with the serious political issues it raises.
Finally, let's not forget the most important thing of all. A science fiction movie should be fun to watch and for that it needs to have all the other ingredients of a good movie. A compelling story to pull us in. Intriguing characters whose fate we must know, and movie makers who respect their audience and have something to say.