Just re-watched this movie and I just can’t help being disappointed by it. I realize that’s heresy since I loved the series beyond any limit of what is considered socially acceptable, but there you are.
Having pondered it a bit for a while now, I think there's no single cause, but probably a lot of tiny one that by themselves would be unnoticeable:
* The soundtrack was done by someone different from the TV show, and made use of no themes from the show, excepting a 15 second audio cameo in the very end of the credit roll. The STYLE of the music is different, too.
* Book and Inarra being absent for most of the movie gave the film a feeling of, as Kirk says, a house who's children have all left.
* It's not as funny as the series was. Granted, it's not as dry as I remembered it being, but there's little character-based zing
* Wardrobe is different. They were mostly cowboy chic in the series, and they're a bit too Gil Gerard Buck Rogersy in the movie. I'm overstating, of course, but what's up with the rubber suspenders and that gay rubber harness that Wash is wearing in the series? Gone are Simon's very severe 19th century suits and razor-ties, and likewise gone are River's clunky hand-me-down clothes and Chinese dresses. Instead she's wearing leggings with bits of metal bolted to 'em. (I'm not saying she looks BAD, mind you, she's got amazing legs, just stylistically speaking, the look of the movie is very different from the look of the series).
* Cinematography is different. They went with a pseudo-documentary style in the series, absent in the film, so we can see how cheesy some of the sets are, whereas the Cameramen-on-acid style of the series hid that.
* Kaylee is actually too pretty in the movie. Don't get me wrong, I totally love Kaylee, and the actress does fine, but part of what made her so adorable in the series is that she actually had some babyfat on her, and she wasn't the prettiest girl in the room, and she clearly liked the occasional hamburger. She looked like a real girl, someone you might actually know. I'm not a chubby-chaser or anything, but when the movie came out she'd slimmed down to borderline supermodel status, and she's better looking that Inarra in most scenes. the character suffers for that. Distractingly pretty.
* Though the plot is good and clever and all, at no time did I believe the resolution to "The River Mystery" that we saw on screen was the one Wheedon had in mind when he started production on the series years before.
* All the fight scenes go on too long.
* The Alliance forces in the movie were in no way similar to the Alliance forces in the series. In the series, their ships were massive, massive, massive things, vertically oriented and essentially cities in flight. Their crews wore steampunky 19th century looking uniforms, and were derogatorily called "Purplebellies" because of the shading. They were intended as a deliberate parody of Starfleet. In the movie, they use conventionally-shaped ships, they wear rather Buck Rogersy uniforms, and act more like a real military.
* The Men With Blue hands are absent (Aside from a cameo early on)
* No resolution of the Shepherd Book arc.
* Direction is flat and mostly un-interesting, nothing to really make it stand out.
* All the fight scenes are over-choreographed, and feel fakey.
* There's a lack of energy and urgency in a lot of the scenes.
I loved and adored the series, I really did, and I wanted to love this movie, but…somehow, I just don’t. And it’s not getting any better with time.