I have found that the discussion rubrics in the canned portion of our course can be a bit too vague for fine grading. I have added some rules of my own to the syllabus...for example...one rubric we use states, "original post that answers the question and submitted by Wednesday" 20 points. "two original posts in response to your fellow students that are more in content than 'I agree', 'You are right', etc." 2.5 points a piece. Not quite a clear and concise explanation. SO I add in the syllabus that proper grammar is required. Also, if it is a science related discussion, they must have the proper citations. The instructor has final say in how all of theses tools are used and I try to take that responsibility seriously.