Some stuff already available by using WPMU plugins:
– sitewide feed
– recent posts across the system
– latest blogs or members
– member list
all over at http://wpmudev.org/plugins.php
Remember, Buddypress is a plugin for MU – mind you, a really extensive plugin. Other sitewide features can be added the same way they get added in WPMU, it’s just a matter of figuring out if what the end result you want is pulling directly from a BP function or a regular MU function.