I think it comes down to an issue of focus. BuddyPress is a super-awesome plugin with a sweeping vision of turning an instance of WordPress into a self-contained social network, with a lot of focus on inter-user messaging, groups, and other things that one would expect of a social network, allowing for large amounts of interaction to happen on the WordPress instance.
The class blogging plugin, in contrast, has a very narrow focus, being something that allows a professor to make WordPress a bit easier to use when blogging as a class. Instead of overhauling WordPress, it just adds some nice features. It doesn’t add any new ways for users to communicate with each other, because we make the assumption that most users will be students and will be interacting with each other in the classroom. This, I suppose, brings up the other big difference between BuddyPress and the class-blogging plugin. BuddyPress is extremely configurable, and can be used to run all sorts of sites. Our plugin basically lets you make a blog to support a class, and not much more beyond that. If that’s what you want to do, though, the class-blogging plugin should prove to be quite useful.
Hopefully that helps. If not, I’d be happy to answer any follow-up questions that you have.
