I am worried about the attitude of some people who claim that down-voters must always explain the reason for down-voting or who criticize the users who vote down posts without explanations. (Here is an example of such a criticism.)
On the Stack Exchange system, voting is anonymous: users have the right to vote without revealing their identities for whatever reasons they have. Down-voters who do not wish to reveal their identities cannot explain the reason because there is no way to post a comment anonymously. This is a legitimate use of the system, and I do not think that users should be criticized just because they down-voted a post without giving an explanation.
Of course, no one should expect that votes without explanation are as convincing as votes with good justification. Because of this, I think that it is wiser to explain the reason when you vote down a post. But that is a separate issue. My point is that as long as we use a system which allows anonymous voting, it is users’ right to vote up or down without giving any justification.