I teach mathematics in a university. I want to encourage my students to ask and answer questions from each other to build a sense of community. Thus the proposal for a homework site.
I want a teacher to be able to open up a sub-site for his/her class. Everyone can view the posting but only the class members may post or answer.
Of course there is already the excellent math.stackexchange.com. But the point of view of a homework site is different. It is a limited site under the supervision of a teacher. As opposed to a site where everyone can post.
The site need not be limited to math. Just about any topic will do. My guess is that a large numbers of participants will come from teachers who look for some sort of well-orchestrated bulletin board.