I think the idea of categorizing comments is good. I would cite as a somewhat-working example Wikipedia's article flags. Wikipedia has a serious problem with deletionism, but they have ways to mark articles in meaningful ways, like too long, or too short, or irrelevant, or excellent, etc. They can also mark individual contributions as "citation needed" (a flag I would really like to have on comments, see
http://news.ycombinator.com/item?id=2423582 as an example of a citation-needed comment getting, I feel, way too many upvotes).