Thoughts:
There's already a fragmentation issue -- If you're reading "Main" the "Recent Comments" only shows you the recent comments in "Main", if you're reading "Discussion", the "Recent Comments" only shows you the recent comments in "Discussion". I've no idea what the Search function actually retrieves and whether it works across subreddits. Until such is fixed, or until an "All" functionality is visibly added, be careful about splitting the forums further.
I feel your division between "epistemic rationality', "instrumental (or normalive) rationality" and "the future" is bad and would cause people to freeze up when deciding where to post what.
Suggestions:
Less Wrong is based on reddit code, which means we can create subreddits with relative ease.
Right now we have two subreddits, Main and Discussion. These are distinguished not by subject matter, but by whether a post is the type of thing that might be promoted to the front page or not (e.g. a meetup announcement, or a particularly well-composed and useful post).
As a result, almost everything is published to Discussion, and thus it is difficult for busy people to follow only the subjects they care about. More people will be able to engage if we split things into topic-specific subreddits, and make it easy to follow only what they care about.
To make it easier for people to follow only what they care about, we're building the code for a Dashboard thingie.
But we also need to figure out which subreddits to create, and we'd like community feedback about that.
We'll probably start small, with just 1-5 new subreddits.
Below are some initial ideas, to get the conversation started.
Idea 1
Idea 2