iTunes sidebar is used for media management, WMP9 and 10 left sidebar is used for unintuitive buttons and right sidebar is used to display the songs in the playlist or album currently playing. Besides iTunes has the sidebar at least since 2003. When was WMP11 released? Your post is total flame bait and I guess I am falling for it.That's not completely true. The new Finder sidebar is inspired by the iTunes 7 sidebar, that's right but iTunes got it from Windows Media Player 11. Really.
My frustrating experience with WMP and winamp due to their bad interface is the reason why digital music on computers never appealed to me. It was iTunes that changed my music related lifestyle. I guess that is why iTunes and iPod dominate the marketplace.