Here's a quick hack to get the forum categories to display as expanded by default on the forum home page.

Edit modules/boonex/forum/classes/Forum.php

Search for the following...

            if (( isset($p['cat']) && $p['cat'] == $r['cat_uri'] ) /*|| 1 == $r['cat_id'] */)
            {
                $this->setTitle ($r['cat_name']);
                $c .= ''.$this->getForumsXML ($r['cat_uri'], 0) . '';
            }

Comment the following lines

            //if (( isset($p['cat']) && $p['cat'] == $r['cat_uri'] ) /*|| 1 == $r['cat_id'] */)
            //{
            //    $this->setTitle ($r['cat_name']);
                $c .= ''.$this->getForumsXML ($r['cat_uri'], 0) . '';
            //}

Now go to the forum home page and compile the language by clicking the compile link.

All done...

Enjoy

/DM