I think that Main posts should be directly related to rationality, and that posts not directly related to rationality shouldn't go in Main unless you have a really good reason to put them there, especially if you intend to write an extended series of posts. Discussion is for interesting content that isn't directly related to rationality.

That said, it's clear that there's some controversy regarding this issue. Regardless of what the Main/Discussion distinction is or what it becomes, I think we could all benefit from having clearer standards in this respect.