Hmm, yeah I saw the explanations why there will be no dislike-button and why low ratings don't remove points. I think it's the direction Julius already mentioned: to keep the experience positive, to motivate just with positive pushs.
It's kind of sad, that criticism of content seems to have such a bad name here and is feared so much. 
I mean, we critic the platform and certain functions here a lot. But when it comes to the user content, it seems to be hard to get people on the wagon. But I honestly don't know how to change that. Well, at least I have nothing new to add, instead I'm going with what I said in the other post: using what is already there for this and leading by example. And don't be afraid to say so, when something sucks, but in a nice, helpful, professional way. ^^