My impression has been that Main is for things that are in the same vein as Eliezer's sequences, while Discussion is more forum-equivalent, for whatever stuff comes to mind or seems interesting today - if you think of LW as a newspaper, Main means you're trying to be a columnist, Discussions are more letters to the editor. Another way of thinking of it is that if someone ever posted an attack piece about LW, I would imagine "That was just some guy posting in Discussion" to be a valid defence, while anything posted in Main would be fair game. (Main postings are moderated, correct?)
Personally, I don't even look at Main.
Main is moderated in the sense that many posts are demoted to discussion, but posting to it does not require explicit permission of the admins. There's also a third section, "promoted," which is a subset of main chosen by admins.
Near the beginning of this year Wei Dai asked why certain people don't post to LessWrong more often, and Yvain replied that:
But Kaj disagreed that this was the actual standard:
This raises two questions: what is the real standard, and what should the standard be?
Because on the one hand, it's not clear Yvain is right, but on the other hand if he is right on the factual question, that standard seems way too high to me. It would suggest that, as John Maxwell says in the same thread, "The overwhelming LW moderation focus seems to be on stifling bad content. There's very little in place to encourage good content."
The wiki sort-of answers the factual question:
But this isn't an entirely unambiguous answer: how many of the five "factors" does a post need to be in Main? Furthermore, it often seems that the "real" rules are significantly different than what the wiki says. Yvain's perception may be incorrect, but I think there were reasons why he (and presumably the people who upvoted his comment) had that perception. Also, Eliezer recently explained that:
This makes me wonder what other poorly-publicized rules there are in this vicinity.
As for what the rules should be, I'm going to limit myself to two general suggestions:
Finally, whatever standard we settle on, I think it's really important that we make it clearer to people what it is. Aside from the obvious benefits of doing that, I've found that trying to navigate the unclear Main/Discussion distinction is itself often enough to make blogging at LessWrong feel like a chore.
Edited to add: In terms of karma I'm currently the top contributor for the past 30 days on LessWrong by a wide margin. I managed this in spite of the fact that I'm in the middle of doing App Academy and have no time (this past week has been an exception because vacation). I take this not as evidence of how awesome I am, but as evidence that way too little quality content is being posted in Main.