Re: Is BuddyPress a sensible choice for a larger social site?
“low development activity on WP MU in general” – the WPMU folks have been on fire lately, not sure what slow dev universe you speak of. I second Jeff’s comments.
As for some of the features you speak of, a simple cruise through Andy’s http://testbp.org would answer those questions. Compared to the mind-numbing admin work required for Drupal and Joomla, BP is a joy. Of course, if you don’t want to do any work under the hood, go with Ning and have your entire experience dictated to you.