If you have a different thread for every movie somebody wants to discuss, or every new band you want to bring up, topics would fall off the front page after a day, and it fosters less discussion. If you go into the movie thread to talk about Bridesmaids, and you see a discussion on what a waste of time Eyes Wide Shut is, you can chime in and dicuss a topic you wouldn't normally have thought to post about. If you go into the music thread to say you heard a song by some hipster band you liked, you can have somebody come along and agree, you see recs by them and you discover a new band.