I'm sure many users have experienced the dreaded "Downvote with no explanation."
This causes many problems, such as unexplained loss of reputation, lack of knowledge of how to improve a post (which brings down the site's quality), and many other things.
I would like to propose a system that will force users under 500 reputation to leave some sort of comment as to why they downvoted. It can have pre-canned comments like the "Low-Quality Posts" queue. It should also have the space for custom answers.
However, to protect the voter's identity, a new section would need to be created. It could be a simple link under a downvoted question/answer, like
View Downvote Reasons. The OP (or a mod) can click this link and view all the downvote reasons, but not the names of the poster. This will allow the post to be improved, and not need the voter's identity to be viewable. (No revenge).
Theoretically, this could also be used so moderators can over-turn downvotes if they feel like they were just spam or evil.