Although appropriate for the help page, I think that the terse language in the comment policy would perhaps benefit from some deeper explanation and community analysis. I think that the line between some of the "should"s and "shouldn't"s is somewhat gray, and that clarification might be in order, even if only to say that the line is intentionally gray.
What is "constructive criticism that guides the author in improving the post", and how is it fundamentally different from "debating a controversial point"?
None of the "should"s seem to permit any manner of discussion whatsoever. Notably, if I have a question about a previous comment, that question necessarily does not fit any of the "should"s which all relate specifically to the OP. Is discussion ever allowed in comment threads?