Apple released an update for iTunes that brings with it some UI changes.
You now see the return of the sidebar that shows your library folders and playlists. The icons for various sections, such as music, movies, tv shows, etc. have been replaced with the previous drop down menu. The current playlist is now a separate buttons outside the play area. Lastly, on Windows, the menu bar is now permanently visible.
According to TechCrunch, the update also includes bug fixes for the music deletion issue that some users were facing, where their entire local iTunes library was deleted somehow. It's not 100% clear, though, if the update does actually include a bug fix, considering Apple couldn't figure out what was causing the issue, nor could recreate it at its end, and it's not listed in the iTunes changelog, either.