Continuing the discussion from Why was the Popular page chosen as the landing page?:
Whether it’s automatic or not, I think it would be handy to have a simple “Filter” button that’d let you select or deselect (including the ones automatically hidden) the categories you’d like to filter out.
Other handy things could be added to this filter as well, like “Has no replies” for instance.
Edit: Also see this topic about adding new querystrings to accommodate various filters.