There are lot's of things that I don't like but that does not mean that I should downvote them. would downvoting lead to the creation of herd mentality out of fear from offending anyone. Down vote pack hunting would also be an issue.
@whitelightxpress said that downvoting should open a screen where you had to check a box for the reason why you downvoted. I'd go further than that and say that you had to fill out a series of check boxes. I feel that if you think that a post deserves a downvote you have to justify it.
If this pop-up was to exist it would have have to allow to only be seen once
Yes, I think it should be a one time thing.