Ther are many factors that make a good movie. Characters I care for and are not just walking stereotypes, a convincing and somewhat original plot even within the boundaries of it's genre, good acting, good editing (bad editing can take me out of a movie really fast, Serenity was a hack job on that department), a nice pace, a somewhat clear direction, not too much emphasis on action scenes to a degree that they overshadow the story, substance over style (this is where Whedon has to learn a few things), the right mood for the setting and a couple of others. Or did you want examples?