A cowboy show in space. It has been tried before. This one lasted one season. It has become our family bonding experience, thanks to a Borders Valentines Day sale on the boxed set.
A thrown-together "family" of outlaws with principles, operating at the edge of the law, at the edge of civilization. It's Maverick again, with recurring women. It's Giligan's Island on a starship.
Why does it work? I invite my readers to help solve the mystery. My guess is that Josh Whedon got his archetypes right; the cast of lovable and exasperating characters are a good inventory of the Lovable and Exasperating Characters that, in other contexts, would have been made into gods. One wants to see what happens when these folks come together. Also, and this is the mysterious part, the show creates a strong feeling of home, of familiarity, of comfort. The spaceship looks like a place one could come home to. The port cities are strange and scary, but not -- as in Star Wars -- in a way that suggests that all hell has broken loose. One other thing: ethics keeps coming up. People wonder about the right thing to do, fight about it even.
Firefly creates an alternative world with helpful connections to the real worlds of its viewers, reminding them of some things. Have a look at it, sometime.