The New York Times in 1989 asserted that the genre is "guitar music first of all, with guitars that blast out power chords, pick out chiming riffs, buzz with fuzztone and squeal in feedback."

Source: Wikipedia (full article)

As someone with a working knowledge of a guitar, I do get the part about the power chords and buzz. But to me, it's essentially "guitar music" (or at times Piano) which sounds different from Classic Rock.