I sat down with my friend in a coffee house two days ago and I took some fountain pen notes for two and a half hours, while she worked on her travel writing work. Here are some of the results.
What are the main development in music in the 20th century? I have no idea, but these are four that I think are significant.
1) Musical modernism, conceived as serialism developing out of classical harmony but at the same time anharmonic. This music is defined against tonality, but also needs tonality for its very definition, since you have to know what to avoid. They are also alien to any vernacular tradition.
2) A classical tradition that cites echoes vernacular or folk idioms, following 19th century effort along these lines.
3) The rise of elaborated vernaculars, like jazz. These vernaculars are modeled on classical music in that they aim toward complexity and sophistication, and develop their own forms of modernism / avant-garde, Neo-classicism, etc...
4) The mass-commercialization of vernacular idioms; the creation and development of new vernacular idioms out of commercial music.
These are not all the developments, just those relevant to what I am thinking about.