Robert Siegel talked with mastering engineer Bob Ludwig about the "loudness wars" as part of NPR's lookback at the last decade.
In the interview, Ludwig argues that modern mastering compression is squashing the life out of music:
People talk about downloads hurting record sales. I and some other people would submit that another thing that's hurting record sales these days is the fact that they are so compressed that the ear just gets tired of it.
The entire interview is worth a listen. Along the way, Ludwig cites the video embedded above, which offers an audiovisual demonstration of what gets lost when music is over-compressed.