An interesting experiment Yahoo (and similarly Google) is undertaking internally is called "Friend Finder". Friend Finder analyzes a user's email traffic and indicates the friends with whom a user has strong email connections based on incoming/outgoing traffic and the frequency and speed in which two parties respond to each other.
Xobni already does this for Outlook: it clusters contacts based on how frequent they appear in the same CC field and other factors.
Gmail also has a list of frequently mailed contacts based on an affinity score. Maybe it can expand the list with more information about the contacts that have the most starred messages, the longest conversations, the fastest replies or the most forwards.
Related: Social Gmail