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:

• SCIENCE ( Discussion of Math, Science, Decision theory, thought experiments, etc, whether LW-produced or discussion of outside research -- the place that the actual mathematicians and computer science theorists would be participating in)
• SOCIAL (Meetups & any discussion of other real-life LW community activities (e.g. holiday rituals, housing arrangements) goes here)
• MEDIA (The monthly media threads would go here, and so would HPMoR, Friendship-is-Optimal, all other LessWrong-inspired fanfics discussion -- but we would now encourage the more easy creation of threads that discuss specific authors, novels, movies, webcomics, SMBC, xkcd, etc. LW regulars would be encouraged to advertise and seek feedback for their own fiction).
• OPEN (A merging of "Discussion" and "Open threads" - every topic that doesn't go anywhere else, and where barrier to entry is low. )