One thing very at odds with Apple's design etiquette with iTunes is there are a lot of message dialogs that block the whole ui. Typically a message dialog that blocked things underneath it would be a last resort, and of all things in a music player app.

One example is if you have a song in your queue and press play on another song you get a message (which of course blocks the UI):

"You are about to start playback. Do you want to clear the song previously added to Up Next?" [Play song] [Cancel] [Clear Song]