I always thought that closing a question was a way to fight spam, homework and well-known questions. However in the last few months I see a lot of questions in moderation queue voted to be closed, while the questions are completely valid, upvoted and probably deserving some more discussion - it is not like the accepted answer is the only and ultimate one without any doubts. To give an example, I just got a notification today that this question was closed today as "not constructive". I don't know what's wrong with people casting such close votes, but I always thought of stackoverflow as a place for discussion, not as a place for a single valid answer.
So, I think that "close" votes for reasons other than "off-topic" or "duplicate" should be restricted somehow. I don't have a solid opinion on what kind of restriction this could be. May be disallowing to cast close votes in "not constructive", "too localized" or "not a real question" categories on questions rated above some threshold could help. May be the threshold should be put on answers. May be threshold may be put on the voter's karma. But definitely "close as not constructive" on questions similar to one referneced above is well, not constructive.