I wasn't a big fan of Serenity when I first saw it, and I'm afraid I'm still not a fan. Watching Serenity (the Firefly pilot) and Serenity (the Firefly movie) back to back merely served to highlight all the gripes I had about Serenity (the Firefly movie). The television pilot (and series) featured an ensemble cast and while the movie started off that way, it eventually turned into Buffy the Vampire Slayer (or, in this case, River the Reaver Slayer) and a bunch of people who she magnanimously puts up with. It didn't help that Malcolm Reynolds, the only other strong character in the movie, seemed to have undergone a complete personality shift somewhere in between Objectsin Space (the last episode in the series) and Serenity (the Firefly movie). In fact, I found Mal's personality shift so jarring that I almost felt I wasn't watching the original Firefly cast but their Mirror Universe counterparts.
I could have forgiven the inconsistencies in Mal's characterization in the movie as opposed to the series (after all, a bit of time was supposed to have passed and people do change) but I really found the resolution of River's story arc to be really difficult to stomach. I may be in the minority here but I find the concept of omnipotent and omniscient heroes to be, well, boring. During the series, while it was obvious that River had some unique and powerful abilities, the fact that she was off in la-la land half the time prevented her from being too obtrusive. Once she was "cured" of her psychological ailments, she became...boring. And obtrusive. And speaking of how she was "cured", it was simply too neat and tidy: River sees a holographic recording of Ms. Exposition describe how an entire planetful of people just decided to lay down and die, how the Reavers came to be and after witnessing the bloody climax of the recording (Ms. Exposition getting raped and eaten alive by a Reaver), River purges her demons by vomiting against a nearby wall after which she declares: "I'm alright...I'm alright". Huh? What? This was most unsatisfying considering everything that she and Simon went through during the course of Firefly's fourteen episodes.
Yeah, we get it. She's cured. And a total badass now
Personally, I think the movie would have been much more satisfying had River died; if she had simply closed the blast doors, tossed Simon's medical kit through the doors before they closed and then gotten killed by the Reavers, the movie would have been much better. We would have been spared the rather difficult to swallow scenes of her wading through the Reavers and piling their corpses up like cordwood and River sacrificing herself to save her brother and the others would have been very poignant, on par with Spock sacrificing himself to save the Enterprise and her crew in Star Trek II: The Wrath of Khan. Her dying to save her brother would also have lent thematic symmetry to their story arc. And more important of all, we would have been spared the exchange between River and Mal at the film's end where we realize that she can essentially do everything that everyone on the crew can (and probably do a better job of it) which leaves us with the question of what purpose they serve now.
Spock saves the Enterprise and its crew in a selfless act of heroism
Of course, having River die in Serenity probably would not have sat well with many Firefly fans, especially since almost a third of the original cast ended up getting killed off in the film. River dying probably would have resulted in grief-stricken Firefly fans converging upon JossWhedon's home with torches and pitchforks in hand and bloody vengeance in their hearts. However, I find nothing wrong with the idea of the hero dying in a story. I thought the Star Trek moviefranchise would have been much better had Spock stayed dead in the aftermath of Star Trek II: The Wrath of Khan (if anything, it could have opened up some interesting story-lines about how Kirk and the rest of the crew dealt with their grief and eventually got on with their lives) and I thought Lethal Weapon 2 would have been a much better movie had Riggs died in Murtaugh's arms. There's nothing wrong with the hero dying, either in the act of saving the lives of others or avenging a loved one.
If anything, it's heroic.