It's interesting that Apple provides so many ways to access media across devices:
1. iTunes Home Sharing -- access the media on your Mac from another Mac or iOS device, and control it from the latter.
2. AirPlay -- Send media (from any source, local, YouTube or an app) to an Apple TV or Airport base station. Similar to the above, but you control the playback from the sending device, not the receiving one.
3. Remote app -- use your iPhone as a remote control for your Mac.
(This is not even mentioning more generic protocols like SMB, sftp, WebDAV, VNC, etc.)
I wish Apple let Macs and iPads receive AirPlay streams. If you're playing a YouTube video on your iPhone and want to see if on a bigger screen, you should be able to do so just by clicking a button.
More generally, there are three roles here -- the device that stores the media, the device that plays it, and the device that hosts the UI. Let any device play any role, to the extent possible.
(reposted from https://plus.google.com/104797602913203220453/posts/WJUES6sotod)