iTunes 10’s interface is a significant step backwards from its predecessor. The monochrome sidebar makes it harder to pick items out, as you only have the shape of the icon to go by. The new album list view takes up too much horizontal space, while the one from iTunes 9 was fairly efficient with it. Elements like the checkmarks and the volume control just look bad.
The worst change, which completely breaks Apple’s set-in-stone UI convention, is the use of vertical close/minimize/maximize buttons. What were they thinking? If any other developer broke the Human Interface Guidelines like that, Steve Jobs would probably personally punch them in the face.
Astute iTunes users could point out the fact that when you used the “Mini Player” (Command+Shift+M) in iTunes 9, the same vertical buttons were used to save space. While that is true, it wasn’t for the main interface that you spend most of your time in. And such space-saving is only necessary in the Mini Player.
Anyway, I won’t bore you with further design ranting. You can restore the buttons to their rightful orientation by entering this command into the Terminal:
defaults write com.apple.iTunes full-window -boolean YES
Re-entering the command with NO instead of YES will reverse the change, should you take leave of your sanity in the future.