I just found a scientific article (http://www.nature.com/srep/2012/120726/srep00521/full/srep00521.html) linked on slashdot (http://entertainment.slashdot.org/story/12/07/30/0143228/study-finds-new-pop-music-does-all-sound-the-same), which investigates the evolution of popular music in the last half century.
Since I cannot paraphrase it any better, and I didn't read it fully, yet, I will just post the abstract here:
Popular music is a key cultural expression that has captured listeners' attention for ages. Many of the structural regularities underlying musical discourse are yet to be discovered and, accordingly, their historical evolution remains formally unknown. Here we unveil a number of patterns and metrics characterizing the generic usage of primary musical facets such as pitch, timbre, and loudness in contemporary western popular music. Many of these patterns and metrics have been consistently stable for a period of more than fifty years. However, we prove important changes or trends related to the restriction of pitch transitions, the homogenization of the timbral palette, and the growing loudness levels. This suggests that our perception of the new would be rooted on these changing characteristics. Hence, an old tune could perfectly sound novel and fashionable, provided that it consisted of common harmonic progressions, changed the instrumentation, and increased the average loudness.