Let me just toss a couple ideas out there for the wonderful, helpful, talented WPMu community to mull over:
Sitewide Twitter updates: I offer Twitter Tools as a plugin option. But wouldn't it be great of the site admin could set up a Twitter account for the entire site, and whenever anyone posted, there was an update in Twitter?
Automatic cross posting: Many has been the time I wanted a post in one blog and anted it to appear on another. Ideally, the site admin to cross post to ANY blog, while other blog owners could post only to blogs where they have author or admin rights.
Site-wide text/executable php widgets: This would let the site admin easily create widgets that would be available for all bloggers on the site.