As we know when others are playing music on one's server or when you are playing music there is a notifications on the right side of the screen detailing what is being played. Now imagine clicking on that notification and having the option to sync with what is being played. Basically you will be listening to what another person or computer is listening to and be exactly at the same point in the song.
So now in the above situation I would be abe able to sync my music playback so all computers are playing the same thing at the same time. On top of that, my friends and I can listen to the same stuff at the same time and I could pretty much make a live music experience for my friends. And for those sentimental guys you and your girlfriend could listen to the same thing, ah how sweet.

And as far as the coding aspect goes, you could have the player send information to the sever when a change is made rather than all the time. So, if I am listen to a song and someone is listening with me, my player will only notify the server where I am a few times. Once when someone elects to listen with me, then after that whenever I change songs, pause songs, or do anything that would cause song playback to deviate from a regular play. I know nothing about coding but that would be a clever way to do it.
::TLDR:: Syncing several computers to the same time in music playback would be cool through Subsonic