Takes your breath away I read a plot summary before seeing this movie and I had my doubts of how it would work out. It sounded like the deaths of Wash and Book were unnecessary, but when I saw the movie, I was relieved to notice that it had been carried out well and they did have a meaning.
When watching Firefly, I often considered Wash a "lesser" character, rather annoying with his jokes, but mostly they were on Jayne and Jayne somewhat deserves to be picked on. Wash carried out his duty, bringing the ship down safely when it was uncertain if they would make it. Also his death marks a sort of a point of no return: even if the ship was in flying condition, the others might not be skilled enough to fly it out of there. Also on the DVD comment track it was said that his sudden death made the whole movie theater burst in laughter. One of the good things about Joss Whedon's productions is that he can make serious things funny (like the torture scene in Firefly). I feel sad for Zoë, though she wanted to have a baby with Wash and unless he donated to a sperm bank in the past, that dream is now never going to come true. The dialog in the end where it sounds like they're talking about Serenity, it's in fact about Zoë and that's very touching. I wish she would find a moment for mourning and somebody to comfort her.
It's unfortunately impossible to have a movie with 9 main characters, even more if counting the Operative and Mr. Universe, so some of the primary characters from Firefly (mainly Inara) had to stay in the shadows to give room for plot advancement. There simply wasn't enough room for all the things that were considered important in Firefly. They couldn't use Inara's status as a Companion to their advantage because they seemed to be chased by the whole Alliance so diplomatics wouldn't have helped. Also there wasn't much room for romance due to the exhausting pace of the film, but at least Kaylee and Simon got each other in the end (I laughed at River spying on them from the ceiling!) and it is hinted that Inara might also stay.
There was quite a lot of combat and I'm not sure if it was quite necessary, but it was tolerable. I'm glad they didn't show River fighting the Reavers because that would've spoiled a lot. You could imagine that she sacrificed herself, and when the door opens for a moment you wonder if it's the Reavers coming (how did she manage to open the door anyway, didn't Kaylee rig it so that once it is closed, it cannot be opened anymore?) but then we see River standing there and before we can even sigh with relief, the wall behind her crashes to reveal the Alliance soldiers. That's just a priceless moment. I don't care how she did it; I'm ready to believe she's a superhero. Mal fighting the Operative took a bit too long but it ended well.
I don't think that Mal threatened to kill any of his crew; he just said that it's the end of the line for anyone who doesn't want to come along to Reaver territory. Staying behind in a slaughtered town doesn't seem like a very nice option but you could still survive. The Reaver territories must be too wide for circumventing and it seemed like there was no time to waste. It wouldn't be much of a movie if it featured the heroes taking weeks or months to sail through empty space to get somewhere.
I'm not sure how Mal knew the Alliance was waiting behind the ion cloud but a lot of the movie is about intuition. He could've gotten away without alarming the Reavers if he had wanted to, but he shot one of their ships just to catch their attention and have them chasing him to provide a distraction. It's another priceless moment when the ships emerge from the clouds because you would realize what's going to happen right before it happens so you could laugh at the Alliance for thinking Serenity was all alone and vulnerable.
I didn't like the character Mr Universe. He's more like someone from The Matrix or like Eyes Only from Dark Angel; not someone space cowboys would befriend. It was clear that they needed some way for the Miranda information to be broadcast but I just started thinking, wouldn't it have been way cooler if they had infiltrated a major Alliance TV station and sent the message from there? But then we might not have the big fight between the Reavers and the Alliance, and it would be a totally different movie.