iTunes 11 Navigation
Disclaimer: I am not a "heavy" iTunes user, but since version 11 came out -- out of curiosity -- I have tried to use it more.
I think Lukas Mathis is spot-on with his comments on iTunes 11's new navigation. Mainly, the different ways to change "views" and "sections":
To reiterate: if I’m in the store section, I don’t click on «Music» to go to my music, I click on «Library». But if I’m in my music section, I click on «Music» (which is on the opposite side of the screen from the previously-clicked «Library» button) to go to my movies.
I may not use iTunes as my default media manager for music and movies, but I do use it every day to check for app updates and buy new apps. Therefore, I am constantly switching between the "Apps" view of the Library and the "My App Updates" section of the iTunes Store (which, by the way, still hasn't been updated to a new look). In the Library, the dropdown menu to switch views (Music, Movies, etc) is on the left; when I check for updates and I'm given the updates page, to go back I have to work with a different dropdown on the right. Maybe I'm too old for this stuff, but this breaks my muscle memory. Even worse, if I am in Apps, I check for updates, and click Library in the My App Updates page, I am not brought back to Apps -- the Music view opens instead. This is incredibly confusing, and it still happens in iTunes 10.0.1.
I recommend reading Lukas' post for a reasonable take on iTunes 11's complicated navigation scheme and menu layout. Personally, I have gone back to showing the sidebar because it just makes more sense to me.