I would to see the merging of the marketplace and social in this capacity: allow users to upload playlists consisting entirely of tracks available in the Marketplace to the Social hub. Users could set the playlist to be seen by their friends or everyone. It would be automatically tagged with genre(s) based on the tags of the content already in the Marketplace. If unavailable tracks are in the playlist, prompt the user or automatically remove them. From the Marketplace search, a Social Playlist category would show user hosted playlists. Searches could opt to see their friends' playlists or the whole community's and view by genre/tags.
This would be a great tool for music discovery and sharing. One example that comes to mind is T.V. shows with lots of music in their soundtracks. A user could maintain a playlist of songs from that show.
An unrelated suggestion is to add an option in the main search results window to hide unavailable content in the Marketplace column. Another is the boost the quality of the Channels by (a) actually updating them and (b) using the available APIs from sites like yes.com, tunegenie.com, and even iTunes and Amazon mp3 to tracking trending music outside the Zune community.