The general idea of this proposed feature is to make the flagging system serve more of an educational and guiding purpose to both the poster being flagged and to the community member that is submitting the flag. I think that being able to better communicate and understand the specific reasons a post might not be acceptable would be more effective than having the system effectively just give a blanket “no” to the whole post. Only saying “no” to a post as a whole can be misinterpreted as saying “no” to acceptable/salvageable content in a flagged post and not just the problematic aspects.
Having more guidance intuitively built into the system would especially be useful given that two of the main objectives of Discourse’s flagging system (as I understand it) are to encourage the flagged posters to edit/move their flagged posts to become acceptable and useful (if the post is salvageable), and for the community of a given forum to be generally as self-corrective as possible with minimal moderator intervention required.