One more odd thing, I find once you make a post in a forum, it sparks others to post. Leaving a forum for someone to start a thread takes much longer.
I just checked your site and I see you have the front page restricted to members. In my opinion, you have to follow the funnel technique of conversion. Let guests see the content. I can't remember the ratio but 1:500 people will move from viewing to registering. Forcing to register is a turn-off for some if they are just surfing. If a surfer can see it though and is interested, he will take the time to register.
The last thing you said I think is a biggie, I get frustrated when sites block their content from guests. I just changed that about my forum actually today, you must have click before I made the change.