How can I change tag separator from "," to "+"?
you can use a hook_form_FORM_ID_alter. I used this validation to make sure users use commas instead of spaces:

function MYMODUDLE_form_FORM_ID_alter(&amp;$form, &amp;$form_state, $form_id) {
 $form['#validate'][] = 'custom_form_validate';
}


function custom_form_validate($form, &amp;$form_state) {
  $tags = $form_state['values']['field_MYNAME_tags']['und'];
  foreach ($tags as $tag) {
    if ((strlen($tag['name']) &gt; 15) || (substr_count($tag['name'], ' ') &gt; 1)) {
      form_set_error('field_MYNAME_tags', 'Please enter tags separated by commas');
    }
  }
}