I like the fact that with Panel, you've focused on simplicity and ease-of-use. To that end, a recommendation, if you don't mind: Remove the zones from the settings and let them live under their own icon, something that suggests rooms (perhaps). This way, you are putting the most important choices out front for the user -- what to play and where to play it. The zones, to me, aren't really a setting, anyway. And this way, you aren't inviting a novice user to start playing with any actual settings.
(I also agree with the people who've been suggesting some sort of user-account control. I want to give certain users the opportunity to listen to music on their own computers and mobile devices but I don't want them to be able to modify my library, play music on my home speakers, rummage through my photos or (if I were using Engen) operate my lights.)
Thanks for all the hard work.