As a new user, I find it frustrating when I put effort into an answer and receive down votes with no explanation why. When a user flags a comment, they are given a few choices to explain why they are flagging the comment and an "other" choice if they wish to provide more detailed information.
I would appreciate it if a similar system were put into place for downvotes - a popup where a person can choose WHY they are downvoting from a list of general reasons and an "other" fill-able choice. This information would be provided with the notification of the actual downvote itself - constructive criticism. That way, people will have an idea of what they can do to improve their questions or answers rather than getting frustrated or fed up with purely negative feedback.