My phrasing was a bit poor. By "off the main page", I don't mean links on the main page to the sub-forums off of it. I mean outside of that main page (which you correctly identified), there are no such forum links. While in a forum/"space", the only option is to return to the forum root to get that list.
It's also an inconvenience to have to always expand the sub-forums (e.g. to access the Eyefinity and Crossfire sub-forums).
Ah! Yes, I know exactly what you're talking about, and it sucks.
The root pages (technically Overview pages), easy to update/modify the UI, and for example add a space tree. The content pages, where you see a list of threads/topics... nope. You are in the weeds. There are a couple of ways to navigate from there, but you have to navigate to get back to an Overview page with a space tree.
Which is the only way to get to other forums. Workarounds? None great but...
* in your browser, bookmark the forums you want
* go to Avatar Menu-> Your Places (still navigating, but you can filter based on your activity)