Idea to reduce the occurrence of negative low-value comments on HN threads

When I look back upon my comments, I find some are more constructive than others. Of my less-constructive comments I think "Gee, maybe I just didn't like or agree with the article".

Allow users to downvote articles. Downvotes don't have to have the same weight as upvotes.

If you downvote an article, you are barred from commenting on it. Maybe it even hides it for you. But have a way to undo your downvote.