I feel like the mechanism probably goes something like:
An appropriate umeshism might be "If you've never gotten a post moved to Discussion, you're being too much of a perfectionist."
The problem, of course, is that there are very few things we can do to reverse the trend towards higher and higher post sophistication, since it's not an explicit threshold set by anyone but simply a runaway escalation.
One possible "patch" which comes to mind would be to set it up so that sufficiently high-scoring Discussion posts automatically get moved to Main, although I have no idea how technically complicated that is. I don't even think the bar would have to be that high. Picking an arbitrary "nothing up my sleeve" number of 10, at the moment the posts above 10 points on the first page of Discussion are:
Which means that in the past week (May 25 - June 1) there have been 13 discussion posts gaining over 10 points. If all of these were promoted to Main, this would be an average post rate of just under 2 per day, which is potentially just around the level which some might consider "spammy" if they get the Less Wrong RSS.
Personally, though, I would be fully in favor of getting a couple of moderately-popular posts in my feed reader every morning.
I'd be concerned about posts like "the rational rationalist's guide" being moved to main. It's an amusing post, but I really don't think it meets the standards I would want for the main blog. And it is quite highly upvoted. I think this shows that just going by upvotes may be insufficient.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.