The discourse forums have always been the "latest" page, this layout worked best when Rust was much lower traffic. However these days with our current organisation size and traffic I feel that it would be a better experience if we had the "categories" page as the default instead.
A single stream of posts can seem pretty intimidating and confusing if you're a new user, where as with the categories larger it's easier to find a topic that you're interested in. The "categories" page makes it easier to tell at a glance whether you're on users or internals. It also provides more incentive for people to create and use categories, since they'll be more prominently displayed for more people.