Even with the discussion section, there are ideas or questions too short or inchoate to be worth a post.
This thread is for the discussion of Less Wrong topics that have not appeared in recent posts. If a discussion gets unwieldy, celebrate by turning it into a top-level post.
I agree for the reasons you've mentioned. Recently there has been a lot of discussion about what LessWrong should be about, and I think this is a good way to resolve that question.
Here's a suggestion: What if we divided the forum into sub-pages by topic?
Agreed with Jack about the nuisance value of splitting up not being worth it for the current level of throughput.
If things do get significantly overloaded, a better (though harder to code) answer would be to allow individual users to define filters, and modify the site code to only display posts whose tags match the user's filters.