[Boone Gorges]

You’ll have to dig pretty deep into the bbPress code to make this work right. bp_forums_tag_heat_map uses bb_tag_heat_map to create the cloud, and it doesn’t take order or order_by as an argument. bb_tag_heat_map then goes to bb_get_tag_heat_map. If you were going to make the reordering happen (without a lot of funny business) it might best happen at the level of bb_tag_heat_map (or a custom version of that function). The list of tags is created like this (see buddypress/bp-forums/bbpress/bb-includes/functions.bb-template.php):

$tags = bb_get_top_tags( array( 'number' => $limit ) );

which returns an array. You could then try using usort to sort by $tags[‘count’] before sending $tags to bb_get_tag_heat_map().