Encouraging people to explain down-votes
As a new user, I'm still learning the "dos and donts" of posting on here. I've seen many questions from new users get downvoted with no clear explanation as to why.
I think it would be very useful if it was mandatory for a user to provide a reason when they downvote a question/answer. This would enable the OP to get a better understanding of what they did "wrong" and to help them learn how to do it better next time.