I've just had one recent comment flag declined on a noisy comment.
This comment was a reply to a deleted '+1' comment, and said, simply:
When you go to post a comment, you see this:
Use comments to ask for more information or suggest improvements. Avoid comments like "+1" or "thanks". (emphasis mine)
So I flagged the +1 comment, which was automatically deemed helpful. I then flagged the remaining thanks to that comment as 'no longer needed'. So... why was it declined?