I think that the time period most of these classic films come from (the 20's to the 60's) and the American studio system represents a very special time and place in moviemaking that will probably never be duplicated again. You had a glut of the very finest actors, directors, producers, etc. all churning out picture after picture in different styles and genres like clockwork. I would keep two things in mind, however:
1. That great personal filmmaking is still being practiced today; you just have to be willing to look for it.
2. We're looking at the cinematic past of our country through rose colored glasses, so to speak. TCM doesn't show the many, many, many duds of the period. Remember, for ever CASABLANCA there's about four PASSAGE TO MARSEILLES.