See: http://xjm.drupalgardens.com/sites/xjm.drupalgardens.com/files/3_role_co...

For better usability, the administration form at admin/config/people/taxonomy_access/role/%/edit should have a fieldset for each role, instead of being grouped entirely into one table. The global default would have its own fieldset at the top, with help text explaining the purpose of the global default. Each vocabulary would also then have its own term selection box and "Add" button.