Sweetspot:
If I find that the point I want to make has already been made and repeatedly so I tend to move on without leaving my own input. The only comments that I don't care for are the self-indulgent ones: "You should have pulled her panties down and spanked her until her ass was cherry red..." At that point I'm thinking, "Go write your own story or take a shower."
Well, I find nothing wrong with a commenter indicating alternatives or additions which he/she would have liked to read, as long as they're stated as mere personal preferences. ("Personally, I'd have liked the girl's brother to have been spanked as well, since I enjoy F/m maternal discipline accounts.")
From an author's perspective, I'll agree that being told what I "should have done" in my story, as though it was a 'mistake' not to have done so, isn't likely to impress me or influence my future writing, although I do at least consider sincere suggestions.
While commenting, I'll occasionally refer to agreeing or disagreeing with other comments, either specifically or generally:
"I'm in agreement with several other commenters that..."
"I disagree with bendover's belief about..."
"I'll agree with islandcarol on the matter of..."
So even if you're merely 'echoing' another commenter, I'd recommend stating it in cyberprint--it lets the author know that more than one reader felt that particular way, and it might encourage that commenter (should he/she happen to view your agreement) to continue commenting.
Very few people object to being agreed with, I've noticed...
--C.K.