I've heard people say that it was not that special, that it was just like an extra long episode of Firefly. I'm actually going to take that further and say it wasn't even as good as an episode of Firefly. Somehow all the awesome action and high drama seemed to squeeze out all the fun of the TV show. In the show, even the most terrible, perilous situations were always fun and full of humour. that seemed to be lacking here as Whedon scrambled to fill the movie with story and action. And, really, the story wasn't all that amazing; no big surprises or revelations.
Another thing that bothered me was the deaths of two of the major characters. My criticism of this aspect is something that could apply to Buffy and Angel as well, so it's nothing new for Joss. To me, the deaths were simply plot devices. Neither of the main characters that died really had a very good death scene. Shepherd Book was just found in the dying state, and got the tremendously cliched final words of wisdom scene (why was he the only one still alive on the planet?); Wash was just suddenly dead (shock value?). Neither death was particulalry epic, and seemed only to give the other characters motivation for their later actions.
And the Simon/Kaylee romance was horribly rushed and smacked of fanservice.
Overall, it was a great sci-fi flick, with fantastic action and a cracking pace, but not, IMO, a fitting finale to a series so full of fun and humour (and we all know it was the finale). I don't know, it just wasn't as clever as I thought it would be.