OK, so Toby and I have talked about this and while we think you have a very valid point, there’s no data to suggest that this is actually a problem. That’s not saying it could be down the track, but at the moment I like how we’re giving greater prominence to the current discussions rather than presenting an organised view.
Ideally we’d have a combination of both — maybe five or so recent topics up the top, then followed by the categories themselves; but at the moment what we have is fine; there’s no need to burden users with categories first-up when what really matters is the discussions, anyway.