The iTunes 9.0.1 update changed (again) the behavior of the green button in iTunes’ window. Prior to iTunes 9, clicking this button would change the window into a miniature player, showing just playback controls and a small info window.
In iTunes 9, the green button switched into a real “maximize” button, as in most other OS X applications; to get the mini player, you had to Option-click the green button (or use Shift-Command-M, the shortcut for View -> Show Mini Player).
In iTunes 9.0.1, things changed again, with the button’s behavior returning to the prior “switch to mini player” behavior when clicked, with an Option-click maximizing the window. Apparently the change in iTunes 9 was not well received (or else it was just a bug; the iTunes 9.0.1 release notes describe the behavior as being “fixed.”)
In any case, if you prefer the “maximize when clicked” behavior for the green button in iTunes 9.0.1, here’s how to get it back. Quit iTunes if it’s running, and enter this command in Terminal (in Applications -> Utilities):
defaults write com.apple.iTunes zoom-to-window -bool YES
Restart iTunes, and the green button will now maximize the window when clicked, just as it did in iTunes 9. To revert to the iTunes 9.0.1 behavior of showing the mini player, quit iTunes and enter this command in Terminal:
defaults delete com.apple.iTunes zoom-to-window
The next time you run iTunes, the green button will again show the mini player when clicked. Thanks to this command, this should mark the end of the debate over the green button’s functionality—regardless of how you’d like it to act, you can now make it work as you wish (perhaps this will eventually show up as an option in iTunes Preferences).