Ask PG: Implementing checkboxes to improve comment quality 14 years ago
I think someone would develop a browser plugin to auto-check those boxes.
You could classify the text using a machine learning algorithm using a training set of downvoted comments, and if the comment is classified as likely impolite- then you could ask the user to make sure their comment is appropriate.
You could have a line of text which said "Thanks for being civil and courteous"