I wouldn't comment on a story unless I basically liked it, and if I thought the theme had some value. I'm very slow at writing and commenting and I don't have much time for it; why would I spend it on commenting a story I didn't enjoy?
Same with me - if I critique something it is a sign that I do like the story. I don't see the need to pop in just to tell people that they suck if I don't intend to help them to improve. And since that is time-consuming, I'd rather do it with writers who I enjoy reading and think have potential.
I've been heavily influenced by a book called "Writing without Teachers" I read years ago, by Peter Elbow. Mr. Elbow described the writer a sort of chemist who was trying to get a certain reaction going within the reader's mind. He argued that the main difficulty a writer had was that the readers mind was a sort of black box, which made it difficult for the writer to find out what worked and what didn't. So the best way you could give feedback to the writer was by giving him your own internal responses to what you read, so the author could find out if the writing was having the effect she or he'd intended. His point was that you didn't have to be a great literary critic to give helpful feedback; you just had to give the writer a glimpse of what was going on in your own mind. A consequence was that a negative reaction on your part didn't mean the writing was necesarily bad; and a positive reaction didn't mean it was necesarily good. Your job in giving feedback wasn't to assess whether the writing was good or bad, but to accurately describe your own internal responses.
The despised "banter-responses" can actually be useful that way.
With that in mind, I think if a reader has trouble with a certain theme or idea, it could be helpful to be upfront about it, so that the writer could take that into account in interpreting their responses. For example, if I just can't stand Valygar no matter how he's written (no-good magic hating barbarian), if I'm upfront about that, at least the author won't worry when their long dialogues involving Valygar never seem to work for me. Maybe there might even be value in getting a feedback from someone who disliked the whole premise of a story. For example, someone who didn't generally like redemption stories might give feedback that would give you a different perspective that was helpful in writing a better redemption story. But of course there'd have to be more to what they said than "I just don't like redemption stories."
Or "you think Evil people can redeem. Muhaha, you naive idiot!" You get the idea.
