Maybe some less forums/sections would help? You've got an absolute ton at the moment, and many of them haven't been posted in even once. If you reduce the number people might find it less daunting and post.
First off, when running a forum you will always have a large number of people that join but never do anything. Some of them are spam bots that made it through registration. Others are people who joined but then for whatever reason never return/post (maybe they forgot your url, maybe they lost interest, etc...)
You'll also have people who's post quality is low for various reasons. (Some people might speak English as their 2nd or 3rd language. Others might not have that great education, either they did poorly in school or they're still actually in school learning. Still others post low quality because they don't actually care enough to take time to post more than that... which is something to watch out for on post exchanges.)
To help encourage posting, you need to generate more content/posts yourself. Members will follow what the admin is doing. So if you post a lot, they are more likely to also. (Plus this gives them a wider selection of topics to post in. Because they might be all that interested in what topics you have so far. )
I think CM30 is also right, you have too many forums that aren't being used. So you may want to get rid of some.
The whole "Popular Games" category could easily be nixed without loss. You could move your game reviews section to your general section. Then those popular games sections could be deleted or folded into one forum in the PS3/360 section (so a forum for PS3 games and a forum for 360 games). Since Forza and Halo are Xbox exclusives so they'll never be on PS3.
The "Call of Duty" category could also be eliminated because you have only 6 posts there.
Your sport and music forums could likely be removed (or hidden for now) and anyone who wants to talk sports or music could just use your general forum for it. (Then if you find a lot of people talking about sports and music in your general section, you can re-open those sections.)
Finally, you may want to consider altering your base skin. The black .thead is rather plain. (Consider adding a gradient to it at least to help it stand out more. Perhaps #000000 fading to #777777 or maybe post a graphic request to see if someone can design something more fitting for your site.)