In Drupal, it only takes a couple hours (or perhaps a couple days) to build the concept of a site-wide content manager into your system. It gets mighty tricky to start parceling out administrative tasks for parts of the site.
With Organic Groups (the module behind OpenAtrium’s groups), the solution to this problem has always been messy. But with the advent of Spaces and Group Administrator configuration, it seems like OA and related sites are within shouting distance of giving Group Managers the power of Content Administration.