I couldn't disagree with you more. The characters were consistent, the loose ends tied up, and the story arc brought to a conclusion. And knowing the work of Joss Whedon, it was the conclusion he had in mind before the series even began; he just had to bring it home faster than he wanted to.
Whedon had to make some compromises to make the movie accessible to folks who hadn't seen the series. I saw Serenity first, and I loved it; Then caught up on Firefly, and I could see how what happened to River Tam, how the Reavers came to be, and some of the other revelations would have been much more of a big payoff if I'd seen the series first.
I'd like to know what you think of as a "betrayal." Especially in light of the fact that the creator of the series was the writer and director of the movie.
The point of the series was that it was about characeters who weren't movers and shakers. They didn't set policy, they were crushed by it. The little people that most sci fi projects ignore. Scrupulously observed throughout the series, that was tossed overboard for the film in favor of a pedestrian "save the universe" plot.
Whedon made a Faustian pact, to give the series a second chance at life. I don't blame him for that, or what he had to do to get it. The story that floated around before the film crashed and burned was that, if it was successful and he got another television series, they were going to pick up where the first series left off and ignore the film's continuity entirely.
If I had seen the film first, I probably wouldn't understand either. But the film was a rejection of the themes of the series.