I'm late to the party and it looks like functionality is fairly defined at this point. However, my limited software design experience tells me you can never have too many use cases to go back to. So at the risk of taking two steps back instead of one, I'd like to offer some additional use cases to Michael's original three. Mainly to reinforce the role of blogging to bring down traditional classroom walls...
USE CASE: Teachers keep parents informed and involved. Teachers keep a daily blog of classroom events. Huge benefit to keeping parents involved and supportive of their child's education. With a four-year old in pre-school, I can't tell you how eager I am to get the photocopied page at the end of the week. Daily blog would be better, along with feedback mechanism. Right now I have to write a note and safety-pin it to my daughter's shirt the next day. As a father who usually can't find his car keys in the morning, where am I going to get a safety pin?!
USE CASE: Subject matter expert shares daily life with students. There have been some notable forms of this on the web where scientists communicate with classrooms while they explore the ocean, etc. Blogging could bring this tool to the masses. Example: Have your city mayor blog for a month to support a civics class.
I think the discussion thus far is supportive of these cases. Clear need for flexible permissions and security.
Finally, in terms of features, I've found "Thermal Mapping" of calendars and posted topics to be helpful in many situations. This color-coding feature is CLEARLY a nice-to-have rather than need to have. You can read more about this and see it in action at http://l.editthispage.com/2002/07/13
