Player Synchronization
If you wish to watch a high definition movie together with your close friend who is far overseas, and be able to talk while watching it, most internet connection speeds will probably be insufficient to stream your desktop directly.
What you could do is both download the same movie beforehand, and simultaneously click "Play" button, and talk over some VoIP app while watching it. However, if you wanted to pause the movie for a while, you might have a hard time re-synchronizing the playback time.
That is a problem...