How to load a user field in template_preprocess_comment()
Drupal already loads the user object. You can see the user object on that function:

function bartik_preprocess_comment($variables) {
  $user = $variables['user'];
  // Full load user
  $user = user_load($user-&gt;uid);
  // Now access your field
  $country = field_get_items('user', $user, 'field_country');
  if ($country) {
    $variables['author_country'] = $country[0]['safe_value'];
  }
  else {
    $variables['author_country'] = '';
  }
  // On your comment.tpl.php using $author_country
}