Thing one. It was different, it wasn't the TV show because the TV show couldn't be the movie. It's funny, how after watching the directors commentary on all three of the LOtR DVDs, I think a lot more about the differences of movie writing as opposed to... well just about every other type of writing. To make the movie work as a stand alone piece, and not just a two hour episode, the characters had to be bigger, and a little bit more of a caricature of the ones you saw on the show. I understand this, and I agree with it whole-heartedly.
Thing two. The death of Shepard Book. I understand this from a story crafting point of view, for Wash's death to have the impact it did, Joss had to set it up. It's about juxtaposition. The Shepard died trying to protect his flock. The Shepard is old, the Sheppard has made his peace with God. The Shepard is redemption. He is a man with a past that is full of blood. He did things that were unforgivable, but he changed. In order for someone to be redeemed, he must stay redeemed, he must die redeemed. The Sheppard represents the noble death, from the last selfless act to the last words of advice.
Thing Three. The death of Wash. What an emotional sucker punch. Brilliant brilliant story telling. The senseless death. The Juxtaposition. (I'll explain in Thing four) Wash, was perhaps the one character that could die in this way. Mal and Zoe are both survivors, they have in their pasts, outlived the odds, either one of them dying would simply be the past catching up with them. Jayne couldn't die, because he chose the life, he knows the risks and chooses to take them. Simon has already risked everything, is already fighting against everything. His death would be noble. Inara is far too worldly in politics, and influence, to aware of the risks and like Kaylee, she represents promise. There is something unfulfilled in those characters that represents promise or perhaps hope. In Kaylee's case her death would be tragic, not senseless, it would be the death of innocence, like the death of a child. Wash, however, has what he needs and wants. He has a woman he loves a job doing what he has a passion for, and every reason to live. He is stable (relatively), he is likeable, and he generally isn't in the line of fire (again relatively speaking). His death needed to be pointless to tell the bigger story. It had to be with out qualification, it had to shock, and it had to be unexpected.
Thing four. What are you babbling about. This is the thing Serenity/Firefly, if you strip away the details and the minutia, has always been about surviving. It's about not letting go, about not giving up, and doing everything to keep flying. In a TV show you can do this over time subtly and gently. In a movie, it has to hit home, it has to strike a nerve, and it has to be more in your face. The two deaths are juxtaposed because they represent both extremes for Mal and the Crew. The noble death, the senseless death. Both are dead, both are losses, and after both you have to keep flying. But this is the brilliance, Joss distilled the essence of everything the show was about into those two deaths. The deaths aren't about those who died, the deaths are about those who lived.