It was charming and funny, having Joss Whedon's best dialog ever, it had great characters, played by actors who really got into their roles without overdoing it and had great chemistry together, the stories were (mostly) engaging and fun, there was a good through-line (plot bits that span episodes,) decent production values, the characters actually develop some over the course of the short run, it was very well directed and the art direction is good... It's just plain fun. If you like SF at all, Firefly is a must-see. It manages to go from silly & irreverent to tense seriousness without feeling (too) contrived or mawkish. It is unique, I believe, in the realm of SF television in its sweetness & realness in the face of being set in a completely outlandish environment. The ship's captain, played by Nathan Fillion, is the lovable rogue. This casting decision was impeccable. The guy is actually quite endearing, but doesn't rest on his laurels. He plays the flaws and imperfections well too, and when it comes time to be a badass, he does that. (At one memorable scene, he kicks a bad guy into an engine intake. He can do violence, he can be a jerk, all while maintaining his role as a capable protagonist. He's no Captain Kirk.)

The first time I caught part of an episode, I said, "Cowboys in space? What? Blech," and I left it there for a few years. Big mistake. Although the superficial trappings are of a "Western-style" frontier, you quickly realize that those elements serve as a kind of physical metaphor, and they fade into the background.

I feel that Firefly is one of the rare instances where the hype is actually proportional to the awesomeness of the show. Some folks don't like it, and that's fine. I can understand not being able to get past the SFish devices. But if you can, you kind of fall in love with it. It's the only TV show cancellation that I actually feel strongly about. If I was Bill Gates rich, I'd throw as much money as it took to get everyone back together to continue that show.