A seemingly minor user interface change inis sure to drive some users crazy.
Apple has removed the the traditional sidebar view from this latest version of iTunes. Previously, if you wanted to switch between your music and movie libraries, you did so in the sidebar. The same went for playlists, the iTunes Store, and connected iOS devices -- all of which were found in the sidebar.
To access the same features in iTunes 11, you'll need to click on buttons along the top of iTunes. The Library and iTunes Store buttons are always present, but the device's button is only visible when a device is connected.
Apple might have suspected there'd be some confusion among users, so it included an option to show the sidebar.
You can show the sidebar by pressing Option-Command-S on a Mac or Control-S on Windows. Alternatively, you can open the View option from the menu bar and select Show sidebar.
Voila! Navigating iTunes is just like it used to be, only now it has a different skin on it.