So, having just recently finished watching Firefly, I have come to a realization about the fundamental difference between creator Joss Whedon's two major TV creations: Buffy and Firefly. I'm sure this observation has been made many times, but as a semi-determined avoider of some sorts of fandom, I haven't seen it before, so I will dispense my wisdom forthwith:
Buffy is, at its roots, an adolescent show. The characters are adolescents, even when they have putatively become adults, and the problems presented them (and their solutions) are fundamentally adolescent. When Buffy was based at Sunnydale High, it worked pretty well, somewhat less so once it left. Buffy herself did, eventually, learn to work with others, but even in the end, I would argue, her perspective was that of the lone wolf, the single Slayer, who may have used others in her work, but fundamentally did her work on her own. This idea, that one can exist on one's own, even when taken in an admirable direction ("I will suffer so that you don't have to") is an adolescent mindset.
Firefly is, by contrast, a fundamentally adult show, even though some of its characters are more childish than others (I still want to be Jayne when I grow up). The characters are not only interdependent, but recognize that (well, except for Jayne, who is still my hero, but not because of this), and try and live within that interdependence, instead of striking out on their own. Even for Jayne, the most independent of the bunch, independence is largely presented as a weakness, and the one time he tries to exercise it, it almost gets him killed. Life in a relatively small ship like Serenity is almost by definition an exercise in interdependence, and Whedon nails this, both in the attitude of his characters, and in the tone of the whole all-too-brief series.