Here's a neat tweak we've found while fiddling around with iTunes 11: Its AirPlay popup panel lets you easily select and control the volume of multiple AirPlay devices at once (a feature introduced in iTunes 10), without having to bother with individual settings windows, and it's now accessible from the MiniPlayer. This way, you can pipe audio to, say, a stereo hooked up to your AirPort Express in one room and an Apple TV in another, controlling the volume on both independently from a window that looks like this:
The AirPlay button used to be tucked away in the lower-right corner of iTunes, next to the Genius button (no longer on the chrome of iTunes 11), and the iTunes sidebar (which appears to be MIA). Now that AirPlay has taken root across Apple's systems and is popping up in ever more hardware accessories, it's good that the tech is taking a more prominent space on Apple's default music player.
Have you found any other handy new functionality in iTunes 11? Let us know in the comments!