I really only have one major issue with the social content sites out there right now. The content sucks.
The problem is social content sites (digg, reddit, etc.) are a great blender mixing everyone's choices into a smooth consistent blend. With a relatively homogeneous group, this doesn't matter so much but as more and more people pile into the system, the worse the results become (i.e. it becomes so popular, no one goes there anymore).
What does this mean? Well, the obvious thing is that the market's still open for solutions. Partitioning users by taste into different sites (as suggested in the starting link) is an inadequate solution (IMHO); but what if you could data mine a facebook-like site. That would be killer.