I think one of the main differences between mainstream music today and older music is that the older stuff has more natural melodic and chordal tendencies. This makes it sound more composed and thus more respectable... but not always better.
Most mainstream music fights these natural tendencies in an attempt to sound more fresh.
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Also, the other main difference is musicianship... both in style and quality.
Most mainstream music fights these natural tendencies in an attempt to sound more fresh.
----edit----
Also, the other main difference is musicianship... both in style and quality.