I saw the Benjamin Button movie today. It was pretty good, all things considered, even if it was incredibly sad at the end. At some point I found out that it was based on a book by F. Scott Fitzgerald, which was a shock to me, probably because I can only name one other book by him. (Granted, they changed a lot for the movie. Put it this way: If you were to sum up the movie in one sentence, that would be all that it has in common with the book.)
One thing really struck me, though. To me, the movie was obviously sci-fi, yet I feel somehow like most of the people in the theater would have disagreed with me. There is a widespread perception that a sci-fi movie has to be fairly shallow, set in the future, and involve one or more of lasers, aliens, and time travel. I consider this a shame, because most things that fall into this category are terribly uninteresting. (I'm making a distinction here between entertaining and interesting: Entertaining's value is while you're in the theater; interesting's value is more when you're thinking about it afterward. Good movies will have both, but the latter is more important.)
I used to be extremely upset that sci-fi and fantasy are lumped into one genre, but it makes a little more sense to me now. There's kind of a gradient between the two, and the major dividing factor is how much of a break the story makes with reality. At the sci-fi end of the spectrum, the world is basically recognizable, except for a small change which drives the story. For example, Paycheck was an excellent sci-fi movie (and book). (The small change is spoiler, so I won't give it away here - read the linked article if you're curious.) At the other end of the spectrum, we have stuff like The Lord of the Rings, where the entire world is replaced.
Sci-fi seems to take up most of the spectrum; stuff like Star Wars is really more fantasy, but it still gets labeled sci-fi. For some reason, most people get their impression of sci-fi from the fantasy end of the spectrum, even though the interesting stuff is generally clustered at the other end. That's what I usually call "good" sci-fi - broadly, it proposes a small tweak to the world as we know it, and then explores some of the consequences. The fact that much of what I consider to be "good" sci-fi won't even be seen as sci-fi by most people will probably irritate me for some time to come.