It sounds like an oxymoron -- a discussion topic that is only one thread deep. But it's exactly what instructors need when they want to assign a quick, clean topic like "Muddiest Points". Post your two muddiest points from today's class discussion. No discussion expected, actually, but a great opportunity to Make Learning Visible by letting students read each other's posts. Another example might be the Weekly Response Paper assignment. Discussions is a great place for this kind of assignment. Students can read each other's responses, but because there's no Reply option, there's no pressure to comment. Sometimes, less is more.