I have to be honest, I'm don't like those two images you have right at the top. I feel it looks cluttered and makes the sidebar looks like it's been pushed in an awkward spot and doesn't look right. You've got the featured topics pushed down, which should actually be the first thing you're showing. You can keep the text links you have up there, but it would be best to do away with the images.
You've got a huge amount of blank space at the bottom that I'm not sure why it's there. Whether it's due to the sidebar, or possibly a hidden section, I would look into reducing that space since it only helps to stretch the page.
Regarding the multiple broken down sidebars, I think you could save some space if you could find a way to merge them into a larger one and see how you can simplify it. This is all for the overall goal of cutting down the clutter and limiting the amount of scrolling needed.
On the design side, while I like the header, the theme just doesn't do it for me. You've fixed it up a bit, but the plain black and white design just seems stale to me. Your forum name is actually really cool and unique, and when I think of Overflow, a really awesome blue theme comes to mind.
Best of luck with your forum! 🙂