Jayne - He was classic Jayne. I would change nothing about him. He was distrusting and violent, just the way I like my Jayne.
River - I liked the way they developed the character. I thought the whole conditioned response thing was interesting. However, because of this, I don't think the Alliance will be finished looking for her, even though everyone now knows the secret she was holding.
Book - Was great. I would have liked to learn more about his past other than, "I wasn't always a Shepard," but I guess I'm out of luck. I was glad he fought to the end to protect the Haven. The only problem with Book is that his death will mean nothing to the average movie goer. We care, because we already care about him from the TV show. Those who didn't watch it will just brush it off.
Mal - Was kind of an ass. Zoe was right with her response to him tossing the guy off to be taken by Reavers. I thought that was very un-Mal. Also, I hated his attitude toward Simon. At the end of the series, Simon and River were part of the crew and there was no question. It seemed Mal reverted to the first few episodes in his view of their place on the ship. There's no way our good old Mal would be pushing so hard again to get them off of the ship.
Inara - Is really going to confuse those who didn't see Firefly. They don't especially explain her history with the crew at all. The wonderful sexual tension/chemistry between her and Mal didn't come out very well, either.
Kaylee - I hated what they did to my Kaylee. Despite everything she'd been through, Kaylee was quite naive. She was a sort of country bumpkin and it made her endearing. Who was this articulate (I mean, one who doesn't use phrases such as "there's nobody can help me"), sex crazed woman? Why was the Kaylee/Simon relationship stripped down to this? They always had such a sweet, caring relationship that was nice and slowly developing. I think a better ending for them would have been to have one of their are-they-or-aren't-they moments for once end in a kiss.
Simon - To add to the Kaylee subject, the whole "my one regret is not screwing you" thing really bothered me. What was up with him breaking River out of the institute? He didn't! He hired people who got her to him in the chryo chamber.
Wash - Shouldn't have to die like a chump. If he has to die (which I'm assuming is because of Alan Tudyk's time commitment to other projects), don't make him go out like that. However, I looooved the audience reaction to his death. I knew it was happening (stupid me read spoilers), so I was waiting. Listening to an entire theater gasp as one is pretty cool.
Zoe - I know Zoe is strong and has been through a lot, but after the final battle was over, I would have liked to see her break down once she was by herself. I mean, this wasn't a war buddy. It was her husband. I think it would have been nice to show he was the one who could make her emotional.
Serenity - Hello! Where's the 10th character? The love of the ship is completely lost in the movie.
Story - I think the story was fine. If the problems above were fixed, I think would be a great damn movie. I liked the explanation of the reavers. I'm extremely glad it was the Alliance's fault that they exist.
I'm anxious to find out if and what they change due to fan response. Any ideas?