Star Wars and a bunch of other classic Sci-Fi movies would be terrible Science Fiction (ignore the fact that Star Wars is a space Western Fantasy)...and I just don't hold to that. Conversely, it you take any Science Fiction novel, I'm sure scientists could poke holes in the science. If they did, would it then be mediocre Science Fiction?
This is an interesting problem, and one that doesn't have a very useful answer. For one thing, I seem to fall back on the Potter Stewart test when considering what is science fiction: "I know it when I see it."
For the record, I think Star Wars is fairly poor science fiction, though the recent episodes are far more egregiously bad than the original three. The Trek franchise is kind of on the border for me. On the other hand, I thought Bladerunner was pretty good science fiction. Ditto Silent Running, although it fails as a movie for other reasons.
So what do I mean when I say this? I think I'm working from a fairly narrow definition of "science fiction" -- not hard SF, but at least somewhat durable. For example, I'd care to see my SF observe the laws of physics, or break them in internally defined ways -- the Trek warp drive is an example. This, incidentally, is where the Serenity movie failed repeatedly, ignoring issues of lightspeed lag, transit time, energy budgets, even the nature of gravity, all stuff that any moderately bright and attentive high school sophomore could have pointed out on a casual reading of the script. The science in the science fiction was abandoned for the sake of plot convenience or visual effects.
They didn't play fair with the rules of their own universe.
Compare to Bladerunner. The rubber science in that movie was deliberately set up -- the entire technology of replicants, for example. The areas that might have been more difficult, such as the references to interstellar colonization, were outside the immediate scope of the movie's narrative and as such were glossed over. It felt probable to me. Gravity worked, equipment behaved like equipment should (or shouldn't) and so on.
There is of course the "expert" problem. What Dr. Mike Brotherton, astronomy professor, sees in a space movie is very different from what I see in a space movie. What a medical professional sees is different. And so forth.
I suppose in one sense the problem is labelling.1 If I look at Serenity, Firefly and Star Wars without an expectation of a strong correspondence to some version of physics-friendly reality, they are all kinds of fun. Great story telling. Mythmaking. Fantasies of the technological age, fairy tales of the age of globalization and the dimmest glimmerings of life in the High Frontier.
But anything with a spaceship in it is by definition science fiction. The presence of a spaceship is almost the type specimen of science fiction.
So when I complain about Serenity, manmela hears me limiting our story telling avenues. I think I'm actually wibbling about literary ontology. I think I'm on to something here, but it is probably only the hoary "what is science fiction" chestnut wiggling in my hand.
1. the_flea_king has a take on the science fiction label which dovetails into this thought process a bit.