Here are some quick ideas. Just brainstorms, not necessarily good ideas:
a way to quickly match up with users who listen to the same songs and artists (sort of like 'your listening is a 67% match with Bob')
community-created collaborative playlists that act as 'albums' on the Zune card (has album art that identifies it as a Zune Social playlist). Perhaps download playlists from other people if you have the songs in their playlists. I'd really like to hear other people's playlists of songs I own. Playlists could be voted on/ranked as well. They could be a bit like the iTunes (sorry!) exclusive compilations. You could even sell this playlists (i.e. sell the individual tracks together as a package).
I'd really like to see customizable albums so that I can have commercially unreleased music that I listen to show up on the Zune card. For example live concerts - they won't ever show up on www.allmusic.com, but I listen to a lot of that kind of stuff. Surely the Zune card could be made to display album artist and song title from the tags if it doesn't recognise the album.
Let messages show up on my Zune software or device - I don't use the social much as Zunes aren't available in my country yet, so I only know two other people with Zunes. If I don't check for messages I'd like to see them when I use my Zune.
Album / artist / song reviews
The ability to rank your favourites - at least to specify my no.1 favourite artist or album in addition to simply selecting certain tracks and artists as favourites.
Podcast integration - recommendations, play counts, etc. I find I listen to a LOT of podcasts now that I have an MP3 player with excellent podcast support.
Concert notifications for when a band you like is playing live near you. Concert reviews...fan-generated blog of a current tour.
I'd better get back to work now, but the most important thing (I don't know if you have access to anyone who can help with this):
RELEASE THE ZUNE IN AUSTRALIA PLEASE! and don't ask me how I got one, I'm not telling.
Regards,