One thing you must consider is whether there is enough folks on such a forum to sustain conversations. One of the things I like about bikeforums.net is that I can browse the MTB forum and get it all. The other approach taken by mtbr.com has brakes, 29er, clyde etc .... I like that as well, as it's easier for a newbie to navigate. But there is something cool about munging everything together.

This is a large problem with directories in general is that they are typically only singly categorized. What is really necessary is a generic category mechanism where a sub-forum is only a "view" and you can create your own custom views. When you created a thread, you would select the categories to which it would apply. So if you were talking about the drivetrain on your downhill bike, you would click downhill and drivetrain (or mechanics). If you were talking about the canti brakes on your road bike, you would select road and brakes.

Something like this was originally planned for the Vista file system. It has apparently been bounced back. But there is no way of getting around the notion that most of our documents belong in more than one file folder and we should not be bound by traditional physical paradigms in digital information systems.