While Western films are an easy example, many other genres developed equally complex sets of conventions. The eerie music in a slasher film, the loss of the girl due to dishonesty in the romantic comedy film, and the spontaneous burst into song in the musical film all are some of the more explicit conventions that we no longer consider except in satires of a genre.
As genres pass the point of maturity, they often go into a stage of deconstruction. With respect to the Western film: Little Big Man realigned the conventions in a politically correct way (or more to the point, reversed the cowboy and Indian relationship), Blazing Saddles turned the conventions into humor, and Unforgiven reversed every convention for the sake of tragedy.
Much debate continues about what makes a genre and what doesn't, as well as the way genres are constructed or deconstructed. Duke University's Program in Film in Video holds an annual symposium at USC to explore this very issue. Entitled "Genre Matters", the one-day symposium brings together leaders in film theory, academicsa, producers and directors - in discussion of various subgenres of cinema. The 2003 event took place May 9th.