Unlike the sterile and impersonal ships in Star Wars or Star Trek (or Stargate, Farscape, etc.) Serenity feels personal and broken in. Just as the eponymous spaceship feels lived in, the universe of Firefly (the original TV series) and Serenity feels somehow more real, better developed and less scripted than the universes of other sci-fi series.
While Trekkies fume in the comments (and I could care less about the inevitable pissing contests), let's move past the excellent writing and the obvious mistake of canceling the series before the audience got a chance to get into that world. There are some interesting issues that Serenity raises in the mixture of spaghetti Western and sci-fi.
Early on, the theme of belief is established. That's not really a spoiler, but I don't promise that spoilers will emerge. After jumping from job to job, the crew of Serenity finds an opportunity to make a big score and escape their habit of jumping from hiding hole to hiding hole. Mid-heist, Reavers arrive. In the series, Reavers are established as rogue ships, running without safety precautions, manned by insane crews ready to visit all forms of horror on their victims. In this case, they want to take and eat their victims alive. Mal, the captain makes some choices of moral ambiguity in the process of getting away from the town. Safely aboard Serenity, the crew each ponders how easy it might be to slip across the gulf and become Reavers, the morally ambiguous actions they each make on every job has made it harder and harder for them to see that line as starkly as they once did. On reaching the religious colony of Haven, Shepherd Book encourages Mal to find something to believe in.
At first, Mal sees that in purely religious terms, but Book clarifies that he means belief in much broader terms. Mal is unconvinced, but events intervene, and he has to leave. In the adventures and battles that follow, Mal encouters the secret behind a band of Reaver controlled space, develops a deeper understanding of personal love and love of his fellow men, and connections between his personal mission, the future of his society, and the relationship between violence and social stability.
Great film, go see it. Can a society without violence survive? Why or why not? Joss Whedon doesn't come down as a Hobbesian, but he certainly argues that violence is unavoidable. If that's true, what separates the regular violence from the excessive violence of Reavers? Belief is part of the answer. There are those who justify their violence with belief in society, its rules, and it's convenient fictions. Others find their own rules, share information freely, and bring others to their own cause. It's easy to see how bloggers might sympathize with one side of that.
On a different note, Nathan Newman points out how union labor got this movie in on time, on budget, and for much less than non-union labor could have done.