So if you choose Music from the drop-down menu, for example, the buttons across the top let you sort by songs, albums, artists, genres, videos, playlists, and radio. All you see in the main window is content from that specific category. Now, you have only a drop-down menu on the left to pick the type of media, and buttons across the top to drill down in each category. When you open iTunes in the full window, the left-side navigation that housed your various media libraries from earlier versions is no longer the main interface to look at all your content.
As such, it will probably take some getting used to, but my initial reaction is positive.
This is the first time iTunes has had a major change to the design scheme since its inception 12 years ago. Still, there's little question iTunes needed a refresh, and Apple appears to be on the right path.