function no_common_usernames($errors, $sanitized_user_login){
    $bad_usernames = array('admin', 'administrator');
    if(in_array($sanitized_user_login, $bad_usernames))
        $errors->add( 'bad_name', __('<strong>ERROR</strong>: This username is not allowed.') );

    if ( empty( $_POST['first_name'] ) )
            $errors->add( 'first_name_error', __('<strong>ERROR</strong>: You must include a first name.') );

    return $errors;
}
add_filter('registration_errors', 'no_common_usernames', 10, 2);