Currently it is possible to vote negative on a question, but its not important, that you write a reason for this voting. I think is very helpful for writes, she has an questions, why the user voted negative. Its not helpful to get just a negative vote. Maybe the user doesn't understand the question, or the question is not useful for other users, no matter what ...
Perhaps it is possible that it is required to put a comment if the user will vote negative. I think is helpful for users.