This one's pretty simple actually
the more serious the movie is the darker the movie poster gets. Also the more serious the movie is the more artistic and well thought out the poster (of course there are always exceptions) "Date Movie" is of the comedy, "or so called comedy", genre. Therefore, the poster is light and simple. Also, in most comedy posters there is a funny tagline. "V for Vendetta" however, is much more dark since the movie is much more serious. The poster tend to get darker and darker as the moive genre progresses into the horror genre.