I think that 90% or more of the intelligible rhythm + note combinations have been employed in the totality of recorded music that exists today. American music, by and large, consists of a riff structure in that there are licks and progressions or what have you that repeat on a factor of 4. These lines repeat every few-30+ seconds. Many musicians will use only 2-4 of these repeated lines in writing the music for a song: verse, chorus, verse, chorus, b-section, chorus, chorus.
It is my opinion, then, that popular music today consists largely of discovery. A guitarist, for example, may pick up his guitar with the intention to write something. He may wank off for a bit and then stumble upon something that doesn't sound too terrible and can be repeated often enough to base a song around it. This is discovery. The riff wasn't crafted, it was happened upon. And most likely the note sequence used can be found in countless other songs the guitarist may or may not have heard. Generally, a subtle variance in rhythm and the tonal elements of the instrument/setup with which it is played are the only hints at originality. With this approach, entire works (songs) are largely a reorginization of existing musical ideas (melodies, licks, riffs, chord progressions, etc.). To me, this is the essence of most modern groups. Motifs, themes, and riffs are borrowed and rearranged and placed with other borrowed (intentionally or not) ideas.
To me, to truly invent or create a song means you approached the instrument with a new musical idea and made it happen.
You may say that the riff approach doesn't apply to improvisational music or jazz and other types of music but I think it does. When I listen to improvisational groups, jazz groups, and others I see the same themes and ideas used over and over again. It makes me sad, really.
