When writing a comment on LessWrong, I often know exactly which criticisms people will give. I will have thought through those criticisms and checked that they're not valid, but I won't be able to answer them all in my post, because that would make my post so long that no-one would read it. It seems like I've got to let people criticise me, and then shoot them down. This seems awfully inefficient, it's like the purpose of having a discussion rather than me simply writing a long post is just to trick people into reading it.
Briefly summarize expected objections and write whatever you want to write about them in a comment to your comment.
Discuss things here if they don't deserve a post in Main or Discussion.
If a topic is worthy and receives much discussion, make a new thread for it.