iTunes has two modes, the full window mode where you get full functionality, and a “Mini Player” mode that shrinks the program down into a small rectangle that still serves basic controls. On the Mac, switching between these two modes was accomplished with the green (unless you use the graphite color scheme in OS X) + button next to close and minimize. For any other window in the system though, this button toggles the size of the window to go full or fit content. iTunes broke the UI conventions, but it was serving a pretty nice purpose so most everyone forgave it. Hitting the + button to toggle between the full and mini players certainly became second nature to me.
Installing iTunes 9 brought along a nasty surprise. Apple had changed the behavior of a standard click on the + button to resize the window, just like every other application window. Getting to the mini player required holding down the Option key while clicking the + button. Not a huge deal, but it certainly interrupted the established behaviors of users.
With the loss of a single click for the mini player, we gained a new shortcut combination: Command Shift M. (Thanks for the heads up @cesart) With the discovery of a shortcut I did what I do with any other shortcut I use frequently. I threw it into Multiclutch. I assigned Command Shift M to the up and down three-finger swipe gesture for iTunes. Now I can switch between full and mini player modes in iTunes with a quick swipe on the trackpad. It’s actually even better than clicking on the + button like we had to do before iTunes 9. This doesn’t help desktop users any, though, and I’m hoping a simple utility or command line hack will pop up letting users switch the + button’s behavior back to toggling the mini player.