I've been thinking about this for some time.
A great feature to have would be Social Networking Integration, where as, members could input their Twitter, Facebook, Linked In,( and maybe a few others) log in information and their activity on those sites, would auto update their profile and the activity stream on your WPMU/Buddypress site.
That kind of function really separates the men from the boys in the world of Social networking and is now a common function of most sites.
I think it would also go a long way to enticing more membership sign ups as people move towards integrating all of their profiles.
What do you guys think?