I don’t know if this is possible with our (or any) version of the software, but what about “tags” for each thread, which would be entered or selected by the OP or by a moderator. This would eliminate the need for subforums and remove the Main Page clutter, while still filtering out all of the junk. To wit:
A Great Debate thread about Sarah Palin, which is tagged with tags such as “election 2008” and “palin”.
A Game Room thread about baseball that is tagged with “sports” and “baseball”.
A BBQ Pit thread tagged with “RO” and “user-directed”.
Tags would probably need to be limited to very hot topics (like “palin”), or topics which are broad enough to allow for filtering (such as “baseball”). It would be very time consuming and inefficient to allow mods or users to add tags for each minute detail, such as “17th century politicians”; those obscure topics could be found through the search feature anyway.
And so, at the top of each forum page, or perhaps in the User CP, an organized list of current tags would be listed, for which the user could filter out their preferred results, in addition to a “Display All” option.
…and it looks like something similar may be an option after all: