I'm intrigued - since you very obviously do have a clue, what does it mean and what are the most important factors to consider when looking at the useability of a website or software application?
You should be able to get that online or from books that cover the topic. Here however are some of my thoughts.
xF is a readymade forum template so things like buttons and layout is standard and most users of VB and xF pretty well know what goes. Pay a UI professional (not a techie) to work out forum categories and sub-forums and do the lay-out. They typically use traffics stats and content research to work it out. As yours is cycle-specific and not an omni site, few sub-forums is better. 40 odd is just too many. It should be around 15 and would suggest 3 main categories instead of current 10 - Cycle Central, Cycle Technical and Cafe and Social. . Do only 4 to 5 sub-forums for each category.
Your very first thread
New to CycleChat? Make your first post here and say HELLO 
in your very first sub-forum in your top-most category is a really poor and sad experience for someone new to your popular site. Imagine a newbie posting for the very first time, takes the trouble and writes a couple of lines to introduce him or herself only to find there is not reply or response from the members. I feel very sorry for them. Some figured it out and created their own thread and members respond. Pointless doing SEO targeting and treating newbies like that. Close that thread. A simple research will tell you that members generally avoid entering long running threads as it is nightmare cover all posts unless the topic is interesting.
I will stop here for time being.