On the first page of forum, have option to list categories in collapsed form without showing a long list of forums in each category. If you have many forums, then only one category may show "above the fold" when a visitor sees your first page. He may never see the range of categories you cover. Unless he scrolls down, he may think the forum is too narrowly focussed on just the one category that he sees.

I see some "minus" boxes on the far right of each category, but they are not functioning for me.

Nevermind suggestion. I see this feature working on the Demo Fireboard forum (demo.bestofjoomla.com)

I still cannot get this feature to work on my site. Maybe move thread to General English Support board.