I'm going to keep this short and sweet (and open ended) as I wouldn't want my opinions to sway any discussion right off of the bat.
However, I've been thinking recently about how forums are designed from a user perspective. For the most part they're highly segmented, things being tucked away in separate categories, it doesn't really promote exploration or spontaneous discussion. Why is it like this? Could we do better?
Obviously, I have my own ideas, but as said in the opening, I'll save them for later.
I agree that "exploration or spontaneous discussion" is lacking. Which is the reason why a forum interface shouldn't be the first thing you see when visiting a site. (ie. domain.com/)
What you can do to help your readers "take action"...
1)__ Set thread subscription to INSTANT EMAIL NOTIFICATION by default - those who don't like it can disable.
2)__ Increase LAST POST TITLE characters. On many sites the title is trimmed to small. Thus, decreasing the value of the "last post title". (the longer-complete the title is the more "action" will take place)
3)__ Make it easy for readers to scan page. (remove some clutter; show more white-space; etc)
If you don't have one create your own portal...
A page with the latest threads will definitely increase "action" by the visitor. Threads could be from a specific forum, globally, hottest, etc - anything that will help visitors take action. This will also improve your SEO.