Yonatan Zunger, the Chief Architect of Google+, posted on Google+ a study of how positive and negative feedback on people's posts in social networks affects their future behavior. Specifically, this looks at how "Data Mining Reveals How The “Down-Vote” Leads To A Vicious Circle Of Negative Feedback."
The evidence is that a contributor who is down-voted produces lower quality content in future that is valued even less by others on the network. What’s more, people are more likely to down-vote others after they have been down voted themselves. The result is a vicious spiral of increasingly negative behaviour that is exactly the opposite of the intended effect.
Yonatan from Google+ said that is why you don't see down voting on Google+ and that YouTube is making efforts to change how they handle down voting.
This is actually stuff which has been known in practice by people in the field for a while, and (for example) it's part of why I'll be damned if I ever allow a "-1" button on Google+. In fact, many of the changes we made to YouTube commenting over the past year have to do with applying ideas like these, in very subtle ways. (That one is going to take longer to take effect, since YouTube has quite a lot of inertia, but we've already observed significant changes in tone).
So now you can go ahead and down vote this story but you can't down vote Yonatan's post on Google+.