Wow, it's been quite a journey to get to this post, hasn't it?
Back to the topic

Firefly is awesome for lots of reasons. The verbal sparring is great. The characterization is awesome. The set is super-solid. The baddies are memorable.
But most of all, Mal. He's written to do like everyone who watches Tee Vee and says, "just shoot the bastard!" wants him to do.
Guy takes a hostage and spits out "Okay, nobody move a-"*bang* dead. He acts quickly and decisively, resorting to the most efficient solution, because he doesn't have the leeway to mess around and hope things work out for the best. None of that "enemies are just friends who happen to want to murder us," crap that permeates the "best future" scifi found in most space-faring shows.
That said, Firefly wasn't really sci-fi. It was a western set in space. Sort of how Bablyon 5 was political and ethical exposition within the backdrop of space aliens.
For my money, no show has really dealt with pure sci-fi concepts like Farscape. Things like, the main character gets replicated, one of the two of them runs off and falls in love with another character, then dies - now that character has this weird "okay, so I fell in love with your dead duplicate, how the hell do I deal with you?" relationship. That's the kind of hypothetical issue that can only be dealt with in science fiction. ...not to mention that, at one point, the main character straps a thermonuclear bomb to his chest and proceeds to wreak havoc throughout an enemy space station. Good times...