When subforums show up at the top of a forum, is there a way for them to be displayed as collapsed? Then, if the user wants to see them, he must expand them?

not that I'm aware of but a good idea

For sure. Since these are sub-forums -- and not as important as the main forum -- it would be nice not to make the user scroll below them to see the posts in the main forum.

If expand/collapse isn't easy, it would be nice if you could define if they should appear at the top or bottom of the forum.

i like both of your suggestions!!

I like the idea; i mean, if you can collapse categories on the main index, you should be able to collapse sub-categories in the postlist...