Norman Lebrecht talks about how Gustav Mahler changed music. An excerpt:
But Mahler is not composing a forest scenario in the manner or Weber or Wagner, or a pastoral symphony like Beethoven and Brahms, or a recherche de temps perdu like Marcel Proust. He is taking a huge imaginative leap in attempting to change the public expectation of what a symphony is, and might be. He spends the first four minutes of the work venturing snippets of sound and atmospheric emanations before he delivers anything that might be recognised as a melodic theme, holding the audience attention by force of will. What’s more, throughout the score he introduces peripheral disruptions, sounds that are blown in from outside the text and texture of the music and threaten to drive it off course.