Well, it ain't a Western.
That, I think, was my only major disappointment with the film. The way the Firefly TV series demonstrated its renegade ethos and its frontier politics by adopting that particular historical genre's trappings was pretty much the most glorious thing about it (aside from the sheer quality and intelligence of scripts and acting, of course) . Not every episode of Firefly could be counted as a Western, admittedly, but it's as much the dominant mode of the show as space opera is.
Serenity, by contrast, is space-opera pure, albeit an unusually dark and disturbing example. The only Western elements come during the first half hour or so, and mostly in the form of scenery. Which is a loss, I think, especially since it also means it can't use the theme song that was so perfect for the T.V. series: the nearest we get is the rather apologetic instrumental version at the end of the closing credits. It certainly wouldn't have worked as a theme for the kind of movie Serenity turns out to be, but it's still a shame.
Never mind. The film we have may not be everything a Firefly movie could have been, but it is quite marvellous nonetheless.
NB: Specifics follow. If you're trying to avoid spoilers for Serenity (and it's well worth doing -- I knew practically nothing about the film before I watched it, and it certainly gained a hell of a lot of impact from the fact), then you won't want to read the rest of this entry. If you haven't seen the film yet but aren't trying to avoid spoilers particularly, be warned you'll be getting very full ones here...
The movie does an excellent job of reintroducing the setting, characters and as much backstory as necessary for the cinema audience (the friend whom R. and M. had brought with them to our showing was a Firefly virgin, and found the story entirely unconfusing) without this becoming too obtrusive for the seasoned viewers. Clever touches, like the way the opening sequence implies that the Serenity crew were directly involved in springing River from her imprisonment, and the way Book is introduced as an old friend who's sheltered the crew rather than as a former traveller on Serenity, simplify said backstory for the newcomers without actually compromising the series' continuity.
In fact, the opening sequence is enormously clever, segueing between locations and settings as it shifts the focus from one character and narrative strand to another, but linking them all in a succession of reveals: the illustrated history of the Alliance turns out to be a hologram young River's teacher is showing to her class; the lesson itself turns out to be a fugue experienced by the adult River during her conditioning; her subsequent rescue by Simon turns out to be another hologram being watched months later by the Alliance Operative; and finally a classic "I wonder where she is now?" hook leads us to Serenity and her crew, although we don't learn of the fugitives' presence among them until everybody else has been introduced. It's a virtuoso piece of storytelling, and Whedon at his best.
Subsequent scenes show us, in short order, the crew's criminal lifestyle, River's telepathy and the ever-present danger of the bestial posthuman Reavers. The only exception to the general smoothness of this quite extensive universe-building is, ironically, the rather clunky introduction of "Mr Universe", the techno-fetishist media savant who seems to have wandered in from the cyberpunk movie next door. It's necessary for him to be mentioned early on so that he doesn't seem quite such a deus ex machina at the climax, but he remains a very obvious plot device.
The film has a strong narrative momentum, as the conflicts with the Operative, River's escalating instability, and even the reintroductions of Book and Inara, lead the crew inexorably towards the Reavers, Miranda and the revelation of just how horribly totalitarian the supposedly benevolent Alliance is capable of being. (All the same, there are plot holes here, too. Why was the whole Miranda experiment apparently being run without any kind of observation, so that the Alliance had to send an investigator later on? Why did the Reavers ever leave Miranda, rather than simply staying there and murdering each other? Why isn't the crew's first thought when cornered by Reavers to find a way to activate River, who is their only possible weapon against them? All of these are mendable by a moment's speculation on the part of a forgiving fan, but irritating nevertheless when they could have been given script fixes.)
On the whole, though, the plot is consistent and thought-through. Indeed, there was rather too much plot for my taste -- it's at character and dialogue that Whedon really excels, and I would have preferred the film to have incorporated some quieter moments, in which both would have had the space to unfurl a little.
As it is, the large ensemble cast which was such a strength of Firefly is rather an impediment to a plot-heavy two-hour movie -- a problem that's often visible (among the many others) in the Star Trek films. Even with the reduction of Book's part to two brief scenes, and the perfectly justified removal of Inara for much of the action, several of the characters remain radically underused.
The antagonist, by contrast, is very well explored (and acted, by the phenomenal Chiwetel Ejiofor). His conviction that in fighting for the Alliance's "better world" he has become "a monster" with no place in that imagined utopia makes him one of the most convincing portraits of fanaticism I've yet seen. As hero, Mal too is drawn in considerable depth (and played to perfection as ever by Nathan Fillion), although the reasons for his hatred of the Alliance are relegated to a minor detail of his background.
It's characters like Kaylee and Inara who, despite Whedon's best attempts to give them functions in the plot, seem like spare appendages. This is unfortunately also true of Wash, which is a desperate shame: those of us who remember his and Zoë's relationship from the TV series will feel her loss so very much harder than the newcomers, who could be forgiven for missing the fact that the two of them are married.
For this reason, Wash's death seems at first to be an arbitrary attention-grabbing move, misjudged because (as with Anya's death in the final episode of Buffy) there's no time for the characters or the audience to react to it. In fact, though, it's a masterstroke: coming after Book's far more obviously heroic death (and in concert with my diligent spoiler-avoidance), it convinced me that any or all the characters might die at any moment -- the kind of trick which can only be pulled off once, but which is essential for the final sequence, where it really looks as if Mal is going to bring down the Alliance only by sacrificing his and his crew's lives. (In the end, of course, he does neither, which is a far more plausible resolution.) And there is emotional response, although delayed, on Zoë's part, in the marvellous subtext to her (I think) final line about Serenity's spaceworthiness: "She's tore up real bad, but she'll fly true."
Indeed, it's in the little touches, the dialogue especially, that the humanity of this movie shines through. Mal's response to the Operative's complaint that innocent people are dying above them, "That's truer than you know," does more to humanise the Reavers and remind us of their horrifying plight than any number of Star Trek befriending-the-enemy episodes could. I also found it enormously moving that, in the very late memorial scene, the crew have set up a fourth plinth, uninscribed, decorated with a Reaver spear, alongside those to their friends. I only wish there could have been room for more such moments among the (undeniably entertaining) fist-, gun- and spaceship-fights.
Serenity isn't everything I hoped for from a Firefly movie, by any means: while it may be by turns as funny, as characterful and as moving as the series, all of these are eventually subsumed in the need to tell a gripping story -- which, to be fair, it also does very well indeed. It is, however, a fantastically entertaining and intelligent couple of hours of S.F. cinema, and frankly the opportunities to say that about something are few and far between.
If there's a sequel, though, they need to make it a proper space Western. With cows.
 I actually found the spelling-out of the historical backstory, which in Firefly was only ever implied, fascinating in itself. Somehow, despite talk of "Inner Planets" and "Outer Moons", it had never really struck me that all of this was taking place in a single gigantic solar system, rather than on an interstellar scale. Of course, eschewing faster-than-light travel along with aliens, robots, time-travel, parallel universes, subspace beacons, beings of pure energy and spatial anomalies is perfectly in keeping with the series' scaled-back approach to its S.F. fundamentals, and this realisation adds something to the series for me.
 Whedon seems to have a point he wants to make about the futility and arbitrariness of death. For every Spike, Wesley or Book who goes out in a predictable blaze of glory, there's a Tara, Fred or Wash who dies shockingly and pointlessly. This may be true to life, but it's a terribly bleak thing to watch. Which isn't actually a contradiction, of course.