It would be great to have the ability to attach a rubric to a discussion, just as we can for an assignment—e.g., teachers could establish categories such as Participation and Careful Composition, Quality of Comments, Reference to Material/Resource...

Teachers and students need a link to a rubric in discussions, just as they do with assignments. As most online classes have discussion boards as part of the grade, a way to grade needs to be added to the discussion if it is being graded.