Playing YouTube videos on the Q is as easy as tapping an icon on the top of the app with a play button showing some waves coming off, toggling external playback, then playing any video you like.
It is easy to see the product developing support for NFC pairing and media sharing as the Nexus Q implements it (and, vice versa -- we can imagine bringing Google TV support to Google's American-made sphere).
You can connect to multiple Nexus Q devices simultaneously and send separate media to each of them individually, so this could be easy whole-home streaming.