I understand the intent, but not sure I like it, because the division can be a bit arbitrary and it feels mildly annoying to make the decision. Currently I don't notice a strong difference between Main and Discussion posts. I want to read them all anyway, so I'd prefer if they all showed previews or summaries :-) Maybe authors could say something like "this is a new result" or "this is a popular explanation" in the preview, instead of making a formal separation. I don't feel very strongly about it, though.
Posts on this forum are now divided into two categories: Main and Discussion. Both are shown on the front page, but only Main posts have a preview; Discussion posts are shown only by title.
The rationale for making this change is that I would like this forum to provide a place for doing research as well as a place for presenting thought-out research to an interested audience, but I don't want the former to interfere with the latter. Thus, Discussion is intended for things like going through the details of a technical proof, or posting a quickly-written argument that leaves many steps to be filled in by the reader; Main intended is for things that are interesting and easy-to-read to a typical person who has been involved in discussions about these topics for a while. (Partially inspired by a comment of Vladimir's.)