I never personally cared for the Genius sidebar in iTunes, but I know that a lot of people used it as a way to branch out into new bands and albums and who were therefore distressed when Apple decided to replace it with the Ping sidebar in iTunes 10.0.1.
Good news, though: early on Saturday, Apple decided to do an about face and bring the Genius features back to the sidebar, albeit with a slight rename: it’s now called the iTunes Sidebar and integrates both Genius and Ping data in one easy-to-access place.