iTunes grayed-out menu
One year later, this is happening to me as well. I'm on 10.8.5 and iTunes 11. I have used iTunes since it's inception and don't think I've had this problem until iTunes 11.

Most menu items are greyed out, like Quit and Preferences, but some work, like new Playlist and most items under the File menu. Otherwise iTunes still works, but to quit, I have to force quit or close down the Mac altogether. I see no open dialog boxes anywhere.