It is important to check if the field is empty. In this case field_get_items() returns false (which sucks).

    $items = field_get_items('node', $node, 'field_countries', $node->language);
    if (!empty($items)) {
      $item = reset($items);
      return $item['value'];
    }
    else {
      return NULL;
    }

I think you can generally be sure that if an item exists, it will be in the zero [0] index. And isset() is a very robust thing. So the following works as well:

    $items = field_get_items('node', $node, 'field_countries', $node->language);
    return isset($items[0]['value']) ? $items[0]['value'] : NULL;

For some field types, the value is not in $item['value'] but in $item['tid'] or something else.  
In the usual case (I can't think of an exception atm), the value will be the first thing in the $item array. So the following will also work, but this is more by coincidence than by design. So personally I would not count on it.  
(If $items is a (non-empty) array, then we can safely (*) assume that every $item will be an array too, and not something else.)

    $items = field_get_items('node', $node, 'field_countries', $node->language);
    return isset($items[0]) ? reset($items[0]) : NULL;

The return value of field_get_items() is by-value, so it is generally ok to modify the $items array. This means you can use array_shift().

    $items = field_get_items('node', $node, 'field_countries', $node->language);
    return !empty($items) ? array_shift(array_shift($items)) : NULL;

(*) Most of the assumptions we make here are not based on language-level guarantees, but just on how Drupal generally works.