Rock and much folk music are built on repetition.
Yes, that is a point I often bring up with my friends who make the claim that classical music is repetitive. I also make the argument that forms such as Sonata Allegro were invented in order to avoid repetition in music. You bring up a good point about subtle changes in orchestration. Strategies used by Bach and his "peers" were ideas such as "Augmentation," "Diminution," and "Inversion" of themes. These ideas are very much demonstrated by his Inventions and are certainly demonstrated in all of his music (specifically fugues). Composers make as many attempts to avoid repetition as they possibly can. Thank you for the feedback!
I've seen mathematical work that suggests that classical music relies on partial predictivity. Schoenberg fails on this account, because of the arbitrariness of his tone rows. The sample I heard with the paper sounded like he had spent time playing high notes, so he had to throw in some low notes. He claimed that he was organizing music through time, but it all sounds unmelodic to me.