The default homepage for a Discourse forum seems to be a feed showing the latest posts, although you can change this to Category view, which recreates the classic forum idiom a bit more accurately in my view:
This is preferable, because in addition to showing the top-level categories to users, you also get the feed of latest posts.
Personally, I think this could still be improved. In this view, clicking on one of the categories takes you to an entirely new page for that category, like this:
I think it would be better if, instead of the categories in the left pane being a bunch of static links, they were 'selectors' that changed what was visible in the right pane to a preview of that category. Here's a limited mockup of what I'm thinking of:
Do you guys think this would be a better layout and website flow? I'm not sure if this exists already as a theme plugin somewhere in the Discourse community, or if it would have to be a custom solution.