Three thoughts:
--Didn't one of "our" old message boards have a link that posted all of the most recent (24hrs. ) threads in all forums, so that a user could see all the new conversation at a glance? Such a feature if available on the new boards would make this somewhat irrelevant.
--A look at the WotC boards reveals the theory of Topic segregation taken to its ultimate conclusion: 100 different categories, with the user unsure WHERE to make his post, for feat that no one will see it. The D&D section on the WotC boards is IMHO hopelessly divided; the player's handbook serves as the "general" category and the other forums get looked at far less. To risk too many separate forums is to risk too much choice for the consumer, if you understand my point.
--If the new server/forums have a sophisticated-enough search feature, then feel free to divide the boards as fine-grained as you wish, because finding new topics related to what you wish is a good idea. For instance, a search could in theory turn up results of ALL new posts in 24 hours, which would help the "casual surfer" greatly.
I myself am for a separate forum for "NON-RPG Topics", or some other inclusive name other than just sci-fi or fantasy - what if a group of 20 or so of us all of a sudden get the "Jonesin' " for the new season of "24" for example?

Popular literature and movies give one thousands of ideas for RPG plots, and there is a wealth to be found if one looks for it, as we all know. But dividing too finely makes this place too hard to search, and that is a proposition that will definitely drive away many users.