Let me start off by saying I read this forum fairly regularly and it's been a great resource for me while building and configuring my Printrbot. However, I feel like some significant changes to the topic structure are needed to improve the user experience.
Simply put, the forum is too large for its active userbase. There's too many topic categories and it's having some nasty effects--information can be hard to find, and it often feels like a ghost town. To remedy this, I have a few radical suggestions that'd probably be a pain in the ass to implement, but pitching the idea doesn't hurt.
Collapse the Hardware and Software subcategories into their parent categories. This would leave two top level topics, Printrbot Hardware and Printrbot Software, to serve as primary discussion areas for any of the hardware and software pertinent to printer operation. An informative sticky under each highlighting common upgrades or software configuration would be a nice touch and help solve a lot of questions.
Add two top level topics for Assembly and Troubleshooting. These are some of the forum's largest and most active subtopics, and should be placed near the top of the forum where new users can easily access them.
Collapse the current General top level topic, rename it something like Forum Rules & Administration, and move it to the bottom of the page. Make a sticky in it detailing forum rules and guidelines, and refer people to it as needed. Ideally this topic won't be visited often, but it's necessary to have a place for threads like this one.
Remove the subregions from the Regional Talk category. Simplify it by continent rather than state/country.
Add a top level topic along the lines of News & Announcements at the absolute TOP of the page. This would be the place to keep people "in the loop" about what's going on in the general DIY 3d printing scene. Links to news articles covering printers in general would be welcome here. There's a lot of legal stuff developing and a lot of new products being released--it'd be great to have a place where that information is localized and discussed.
Reduce the current top level category Project Talk into "3D Models & Design" and "Project Showcase". The "my first blob" category is a nice touch too, probably save that one.
Simplify the marketplace by combining everything into one Marketplace category. Buying and selling happen in symbiosis, they should happen in the same place. Make a sticky here and institute some rules on thread titling (like FS/WTB/WTS/FT). Requiring a listed price on any for sale post prevents weird auction style sales. Using some sort of reputation tracking tool like http://www.heatware.com would be a good way for folks to exchange with confidence.
I've said a lot of stuff so I'm going to "graphically" show what the above changes should look like (roughly).
News & Announcements (blue title bar)
News & Announcements
Printrbot Talk (blue title bar)
My first blob
Assembly Discussion
Hardware Discussion
Software Discussion
Troubleshooting
Project Showcase
3D Models & Design
Marketplace (blue title bar)
Buy/Sell Marketplace (w/ titling conventions, no discussion threads allowed)
Marketplace Discussion (Conflict resolution, various discussion threads, no naming convention)
Regional Talk (blue title bar)
~~Subcategories for each continent~~
Forum Rules & Administration (blue title bar)
Forum Rules/Administration
These changes will take us from 35ish categories down to ~19 and condense discussion to make the forum easier to use and making it feel alive.