http://i.imgsafe.org/c0dc8b18a2.jpg

As a newbie here, I'm curious as to everyone's thoughts on the "proper" (that is, the socially acceptable) use of down voting.  When in your mind does a post deserve to be down voted?  Is it acceptable to down vote just because we disagree with the sentiment expressed therein, or is that more akin to censorship?  Should down voting be limited to only or primarily instances where the post is sloppily done or plagiarizes content or doesn't add anything of substance?  

And, if you down vote something, should you offer the courtesy of an explanatory comment, or just move on?  If we are trying to build a culture of cooperation, I think pausing a second to explain your down vote would be beneficial, but I'm interested in everyone's thoughts.  

Also, according to the white paper, down voting "resets your voting power".  What exactly does that mean?  I understand what voting power means (or at least I think I do), but what does "reset" mean?  Does it reset my voting power to zero?  To 100?  Obviously, I'm confused.  

I appreciate everyone's feedback.

Sean