I'm not an expert in this.
However, you have 2 home links, the site looks very dull in theme and your front page is confusing. If you had never come across a forum before how would you know where to post?
Our site took a nose dive after installing VB4 Suite and enabling everything, it confused people and quite a few left. We have now stripped everything back to forum only, given it a clean style so it's simple for people to navigate.
The other things I did was to create a facebook page and app for the site, did the same with twitter, everything that gets posted on our site now gets pushed to Facebook and Twitter. This will help your pagerank too.
You've got far to many Forums, if you're not that busy then you should try and merge as many none "essential" forums into one. What have you got there, roughly 30 or so? EEK
I would honestly suggest stripping everything back basically, get back to basics. Go through your plugins and decide what is absolutely essential and what doesn't really matter and remove those.
Hope this helps.
Here's our site stripped down.
www.the-sps.org