It used to be worse with the whole games section flooding the unread topics at some point.
Firstly, regarding the technical issue:
When you click "Show unread posts since last visit", I reckon you need to put up with it. Because you're basically clicking that - unread topics. If it's full of stuff from "General Discussion", that's really just unfortunate. You wanted to see (pretty much) everything you haven't been updated on, and they're giving you that.
On the other hand, for the VCE and HSC specific ones, I reckon General Discussion (and any other potentially unrelated materials) just needs to go. Journals should be fine, because they give insight onto another HSC student's experience. But other non-academic content is probably still unsuitable.
For that reason, potentially the <Show Unread> subsection needs to be given a small drop-down menu EDIT: or a checkbox menu. Not something I always thought of (or I would've brought it up sooner), rather my reaction to this thread. This could potentially be a nice thing to have in the forum overhaul (well, redesign) but it would probably be more efficient in alleviating the "non-academic content spam".
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Now, the concern of the state of the forum.
For someone like me this has been troubling as well. I've noticed for a long time now that more often than not, the extracurricular sections have been given HEAPS more attention post-count-wise than the academic sections.
Your post raises this important aspect of the forum's state, but we shouldn't be eradicating it either. Because without all of those sections, it's going to easily backfire and turn everything mundane. One of the most important reasons why those sections exist is to promote the 'community' vibe that we still want to develop between members; we're here to help one another but also just for fun.
I'm also a tad concerned that the timing of your post has had something to do with it. I know, for a fact, that the HSC math forums completely exploded for 2 years in a row during the lead-up to the final exams. Now that it's December (and approaching the holiday season), the decline in academic-related posts is to be anticipated. At times like these, there just isn't enough opportunity for the forum to actually appear more academically oriented than otherwise.
Plus, the academic sections are certainly not ignored yet. Sure, I just look at one section mostly, but I definitely catch glimpses of many people contributing heaps to other threads.
Those are my thoughts, and I probably ended without a proper conclusion (because I got lost).