gistfile1.txt

Have two taxonomies. Both work in WP admin, and can be displayed with get_the_term_list(). But one won't work with wp_tag_cloud(). 

Working taxonomy:

add_action( 'init', 'create_post_flags' );
function create_post_flags() {
	$labels = array(
		'name' => _x( 'Post Flags', 'taxonomy general name' ),
		'singular_name' => _x( 'Post Flag', 'taxonomy singular name' ),
		'search_items' =>  __( 'Search Post Flags' ),
		'all_items' => __( 'All Post Flags' ),
		'parent_item' => __( 'Parent Flag' ),
		'parent_item_colon' => __( 'Parent Flag:' ),
		'edit_item' => __( 'Edit Post Flag' ),
		'update_item' => __( 'Update Post Flag' ),
		'add_new_item' => __( 'Add New Post Flag' ),
		'new_item_name' => __( 'New Post Flag Name' ),
	);

	register_taxonomy('post_flags','post',array(
		'hierarchical' => true,
		'labels' => $labels
	));
}

Broken taxonomy:

add_action( 'init', 'create_test_taxonomies' );
function create_test_taxonomies() 
{
  $labels = array(
    'name' => _x( 'Test Tax', 'taxonomy general name' ),
    'singular_name' => _x( 'Test Tax', 'taxonomy singular name' ),
    'search_items' =>  __( 'Search Test Tax' ),
    'all_items' => __( 'All Test Tax' ),
    'parent_item' => __( 'Parent Test Tax' ),
    'parent_item_colon' => __( 'Parent Test Tax:' ),
    'edit_item' => __( 'Edit Test Tax' ), 
    'update_item' => __( 'Update Test Tax' ),
    'add_new_item' => __( 'Add New Test Tax' ),
    'new_item_name' => __( 'New Genre Test Tax' ),
    'menu_name' => __( 'Test Tax' ),
  ); 	

  register_taxonomy('test_tax','post',array(
  	'hierarchical' => true,
  	'labels' => $labels
  ));
}