It reminds me a discussion about the signal/noise ratio. Interesting things came out of it.

Based on this, for my part I don't always explain when I downvote beccause it would waste node only to say : offtopic, innapropriate language...

However I try to send /msg to the person explaining why, when I downvote.(I /msg to thanks for answer/comment too)

I ++ you beccause I really think that it's an interesting point :

If downvoting is an educating action, then you have to do it correctly and explain why you downvote someone.

But to be honest I think that we mustn't waste our votes downvoting whereas there are so many nodes to ++...