It's not a good movie.
I'm far enough removed from Firefly at the moment that my auto-movie-watcher mindset kicked in. By 'not a good movie' what I'm saying is, it doesn't stand on its own. You need to have watched Firefly to appreciate the characters, and without that foreknowledge, the film fails to engage. None of the major characters get enough fleshing out to mean anything to anyone who doesn't know the series.
Case in point is Shepard Book. Lots of films have had the same idea - heroes seek sanctuary with an old friend, old friend gives some advice, gets killed, usually gets to utter a few last words to the hero that strengthen his resolve. It's an old formula that is used because it often works. It doesn't work in Serenity. Book is just some old dude who croaks. I felt more emotion for the woman on the vid report explaining what happened on Miranda.
Wash gets almost nothing to do for the film, and then dies. To a general audience member it's just 'oh, the funny pilot is dead.' You haven't been given the time or the chance to get to know him, to care about him.
Those are the two biggest examples, there are many more. And I know that I telling some of you something you already know. I mainly had to get it out of my head or it was going to keep me awake tonight.
For the record, I love Firefly and I love Serenity. It's a great season 1 finale. It's just not a good feature film.