Have A Moment

There are a few things which make a movie worthwhile. One such element is the presence of at least one moment or situation in which something completely unexpected happens - perhaps something you should have guessed but didn't, or something that was cleverly constructed in the background that suddenly leaps into place. The most effective will lead you to believe you know what where the story is going - then it abruptly does something else instead.
These moments change the course of the movie, and open up a whole world of new possibilities. They hook into your mind, and are what you remember most when the movie ends.

As an example, I watched the animated film Madagascar recently, and was quite ready to call it a complete waste of time when there was a moment. One dark, scary moment in a movie mostly full of poor humour and cliches that just leapt out, unrestrained - not presented as a joke. While this was certainly not enough to make me want to buy the movie, or even see it again, it did hook the movie in my mind - and make me not want to burn every copy I get my hands on.

The moral of the story is: Go see Serenity. And anything and everything written by Joss Whedon. ;-D