Some shows, they overstay their welcome. They are kept on air long past the point where they stop being enjoyable, and wind up only enjoyed by a couple of hard-core fans who didn't give it up in earlier seasons. The type of thing where people will look in surprise and say "Oh, is that still on?" Sometimes, things will experience a very sharp drop in quality between the first and second seasons, a la Heroes, or a gradual decline over several seasons. Something that gets shunted around from time-slot to time-slot, and when it does finally die, leaves most people with more of a memory of the disappointing last few seasons than the awesome start the show had.
And then, there was Firefly. In a scant 14 episodes and a movie, it caught the imagination of people everywhere. It was a brief, bright spark that went out suddenly, and way to quickly. Some people are probably ready to come at me with pitchforks, so let me point out - I meant "almost" for a reason. I would have loved to see what Joss Whedon could do with 8 seasons or more. How the world would build, new characters be introduced, and sub-plots tie up.
Why do I say this? Well, for one, it never got the decline that other series do. Because there are so few episodes, all are both intensely quotable and memorable. There's no "bad" Firefly episode, exactly - I like them all.
Between the series, the movie and a selection of graphic novels, people have really been able to let their imaginations run wild. Since there are less established characters and plots, writers of fanfiction have been able to let themselves loose on this world practically since it's inception. Up there, I said that I'd like to have seen what Joss Whedon could do with the world. Well, we've been doing that. Whether we keep it inside our heads, or share it with other fans, everyone has a different idea about where the story would go.
In fact, because of the story surrounding it and it's cancellation, I'd bet it has a larger and more dedicated fanbase than it might have had otherwise. There's never a point where half of the fanbase lost interest. Instead, the fanbase has only grown over the years, with more and more people being introduced to it.
Also, it's easy to get people to watch it. 14 episodes and a movie isn't as hard a sell as, say, some anime series. Once it's finished, you'll always have something to talk about. If you meet a fellow fan at a convention, too, you already have something in common.
Finally, it gave Joss much less of a chance to kill off our beloved characters. Were any of those deaths in Serenity actually FUCKING necessary?