I was prompted to update iTunes today, but little did I realise what was in store!
A new feature called Genius looks at your tracks and can suggest playlists based up any track in your library that you select. And the Genius Sidebar links off to iTunes Store where you can purchase other tracks by the same artists that you don’t have, as well as other related suggestions.
It’s optional, but I think it’s a nice addition. Of course, I expect it’s sending info about the tracks you play off to Apple, so if you’re concerned about that then it’s probably not for you.