It's a bit unclear for me as well, but I've taken the "Discussion" title to imply that at least posts that are intended to primarily poll people for their opinions, like this one, should go there. I think that, as a rule of thumb, articles in "Main" should mostly be able to stand on their own, in the sense that you'll gain something even if you read just the article itself and none of the comments. (And yes, I've sometimes violated this rule myself.)