Edit: For context's sake, I should say that I have watched the first two disks of the four-disk Firefly episode box set.
Why was I originally not sure? Well, it has to do with the way the movie was built. Joss Whedon did not build a fairy land of science fiction where every ship is tight and every person honorable. To bring that point across, he had the film shot from the most real of camera angles. The audience saw the universe as a person standing in the same space/time as the characters saw it (with a few exceptions), and while some of it was shot from a 3rd-person perspective, at other points we could have been seeing exactly what the characters themselves saw. Unfortunately, I am not one to enjoy things jumping out at me, or character hallucinations that involve monsters or humans acting monstrous (see my review of Batman Begins for more on that rant). So I had quite a bit of trouble with the view from River's mind as she flashed back to school and then to her mental torture. I also had issues with the dead people on Miranda, though oddly enough, the Reaver attack captured in the holographic report did not bother me as much as I expected.
The music was definitely a positive contributor to creating the moods - scary, wistful, etc. However, the music itself was so unmemorable that it had to be a deliberate choice of the producters. There was no Firefly theme song for the credits, and nothing else memorable either. It's not music that makes me want to purchase the soundtrack for the car.
Mal was a character of solid consistency, and I mean that in both senses of the word. Mal was a solid character, and he was very constant in presentation of that character. As I commented above, he was not larger than life, not a hero ("You know the definition of a hero? A person who gets other people killed"), and very very mortal both physcially and mentally. I could really sympathize with him in his interactions. His comment to Shepherd Book that he didn't want to hear Bible stories was entirely credible, as was his acceptance of the plot-wraparound about belief at the end of the movie. And I like Mal's (Nathan Fillion's) face. His smile is entirely self-ironic without denying the other person's worth. If that makes any sense.
I had to get over seeing Jayne as a trumped-up version of Ben Affleck before I could really appreciate him as his own actor/character. The fact that he could move his body to reflect emotion and character as well as his face muscles helped a lot. (And no, I've never seen any of the many movies he's been in.)
The humor in the script and in the more tense situations was well presented. Inara's muttered "Spend an hour with him." during the lead-up to Mal's fight with The Operative was pointed and funny. Jayne in general provided the comic foil for much of the moving-along-now dialogue. Mal's sarcasm, and at times, refusal to give into a smile, caused me to miss the humor of some of his lines until the next one was delivered. Yeah, yeah, sometimes I catch a joke bomb and it goes off later. But sometimes I do catch them at the moment. "...we may experience some slight turbulence, and then explode."
One of the most consistent aspects of the script was the deliberate retreat from stereotypical conversations, and the head-tilting appreciation I got from it. Example 1: Mal and Jayne had an argument in which Mal finally asked Jayne if he thought that he could run the ship better. Jayne's affirmative blew the wind out of Mal's sails, and Mal visibly deflated, rather than pulling some sort of snappy larger-than-life rejoinder. Example 2: When Kaylee and Simon had a tender moment in the middle of the climactic firefight, Simon expressed regret about not spending any quality time with Kaylee. She brightened up and I braced for some sort of sappy "So you love me?" comment. No, no, nothing like that. "You mean... like, sex?" Had that line come out of Jayne, it would have been stereotype. Out of Kaylee, it was surprising, funny, and entirely true to the character. Despite Kaylee's sweet charm, I really think Wash was the innocent in the crew. His open-faced, pleasant demeanor was a fun version of comic deadpan. "Oh God Oh God we're all gonna die." is not usually said in conversational tones.
That is not to say that the movie did not pay any homage to stereotype and plot expectation. The wonderful shot of Serenity-turned-Reaver headed for the League's ship with all the big Reaver ships appearing a moment later was... expected. And I enjoyed it. That was a mental-cheering moment, when we all heard the psychic Serenity "Gotcha!" followed by the obvious "HOLY-!" from the League crew. ("Will SOMEONE just FIRE!" was funny, but really pointed out that reality isn't hair-triggered all the time. Even the hot shots can get caught in a blithering panic.) Later on, when the robot explains to Mal where to find the emergency transmitter, I knew with enough certainty to bet on, that the robot would tell the exact same thing to The Operative when he came through moments later. And the stereotype went screaming to the "enough already!" stage quickly thereafter. As soon as the scene opened on the generator and transmitter tower with no walkway in sight, I leaned over and whispered "cue the James Bond music." I swear I saw that same fight scene before... Goldeneye, anyone?
For a lot of the movie, I kept thinking I had seen The Operative before. No, no, I hadn't. Chiwetel Ejiofor is no one I've ever seen. (No, I never saw Amistad.) I had him mixed up with Tom Lister Jr. from Fifth Element. In any case, Chiwetel was a fine actor for the role, if a bit ponderous in bulk for some of the fight scenes.
And speaking of fight scenes... The choreography for River was impressive. I won't say that I totally believed a 5'8", 110-pound ballerina could take on an entire room of bar toughs, much less an airlock of Reavers. But whoever built the scenes worked really hard to incorporate her ballet background and use her grace and flexibility to add sizzle to spins and kicks. And the spiderman stunts at the beginning were pretty funny, too.
Oh, and Zoe. She was a good character, but not well developed. She played the stoic and sarcastic first mate well, and I enjoyed her presence on screen. She was competent, intelligent, skillful, and her stoic acceptance of loss was underplayed with smarts. I never saw the other Matrix movies, so I don't have anything to bounce against for her.
I may add more to this later. Maybe not.
And if you are suddenly hankering for more Serenity flashbacks, go to the IMDB Serenity quotes section.
I may just have to go see it again sometime. Or buy it when it's out on video.