A user brought this idea to me. It may, in many applications, be valuable to have users join a blog to a particular group (as created by the admin or other users?). This is very different than categories.
I know how/why this is valuable to TeachFor?.Us (region of the teacher), but let's make this valuable to another implementation: classroom-based blogs.
A prof creates a blog site for her history class this year. Each student makes a blog for tracking news events throughout the semester. Next semester a new group of students comes along. After a while... how can anyone go back and review the collective work of the first semester's group of students? Groups would be for blogs as categories are for posts.
Another good use: political blogs. Let users self-identify into a particular political leaning. Then one-click listing of all "Libertarian" blogs are available.
Rather than developing a plug-in to lay on top of the data, this could just be a feature turned on or off by site admin. Users can self-select into a group when creating a blog. This can be managed by one extra data field in the blog table and made useful by adding some simple coding to the portal (a hyperlinked list of current groups would generate a new page of blogs in that group when clicked). I know it's not "this simple" (i.e. reg form updated...), but it's not as out there as some other concepts I've had.
Please don't shelve this too immediately or push way way off. On a scale of 1 to 10 of community minded features that are useful and make a large scale installation much more reader-friendly, I think this one is up there.
[This is not meant to be associated with the idea of site-wide categories which is a whole different game]