I dunno. I just don't know. I'm not accustomed to becoming emotionally invested in fictitious characters, so the fact that I'm crying as I type this leaves me at a loss.
The story is great, and expands on what we've already known nicely. If this is the first act of a trilogy then it's likely that Joss has a plan- an overall story arc that requires motivations for the characters. And of course, one of the strongest motivations would be the death of loved ones.
This movie, while great, was downright painful to watch. This hurt. When the hell did I start caring about these characters as though they were real people??? As femetal noted, Joss isn't one to grant happy endings without exacting a toll. But damn, three? By my reckoning, you've got Book, Wash, and Serenity (never mind that they somehow got her back up and flying, the crash was a death scene for her). It's good to know that Joss isn't afraid to take his story into less-travelled territory, that's a whole lotta carnage on top of what is already a hefty body count of incidentals. That's a whole lotta pain inflicted on the non-fictitious fans. Unless there's more to the story- something justifying or necessitating the slaughter, then there's no point to it.
My big concern is that people seeing Serenity without already being familiar with Firefly won't adequately grasp the emotional impact of what's happening, while the die-hard Browncoats will be so affected by it that it'll overshadow the rest of the movie, which is really quite good.
There were two things in particular not relating to character development that I thought were great. They explained the origin of the Reavers, which now makes a lot more sense. They also verified unequivocally that "the 'verse" is all contained within the same solar system, which supports the "there's no FTL" argument. Of course, this raises questions about the likelihood of a single solar system having hundreds of planets or moons that can be terraformed, but that's less of a stretch than most FTL theories in sci-fi.
Oh, the introductory sequence also filled in a lot of gaps nicely, and River- standing back-lit, drenched in blood, and holding a dripping axe and knife- was dead sexy, like a cross between Beowulf and porn. :-)
Yeah, I'm gonna need to see it again. First, though, I think I need a couple of days to recover and pray that this isn't the end of the story.