How can I add a link to show/hide threaded comments?
You can do something like this:

In your theme's template.php file:

/**
 * Override or insert variables into the comment wrapper templates.
 *
 * @param $variables
 *   An array of variables to pass to the theme template.
 * @param $hook
 *   The name of the template being rendered ("comment" in this case.)
 */
function THEMENAME_preprocess_comment_wrapper(&amp;$variables, $hook) {
  // Add jQuery UI accordian to hide the comments in.
  // It is already added in preprocess node but it is safer to also have it here
  // and doens't add much overhead as drupal will only add it once.
  drupal_add_library('system', 'ui.accordion');
  // Add some custom js to collapse comments.
  drupal_add_js(path_to_theme() . '/js/collapsi-comments.js');
}


In collapsi-comments.js (in this case it is in a folder called "js" in your theme.

(function($){
  Drupal.behaviors.collapsiComments = {
    attach: function(context, settings) {
      $('.comment-form-wrapper.accordion').accordion({
        collapsible: true,
        autoHeight: false,
        active: false,
        header: 'h2.title.comment-form'
      });
    }
  };
}(jQuery));


In your comment-wrapper.tpl.php in your theme wrap the comment form in a wrapper div like this (if your theme doesn't have one and you have a base theme that does, copy it from the base theme  and modify it, otherwise copy from the comment module and modify it):

&lt;?php if ($comment_form): ?&gt;
  &lt;div class="comment-form-wrapper accordion"&gt;
    &lt;h2 class="title comment-form"&gt;&lt;?php print t('Add new comment'); ?&gt;&lt;/h2&gt;
    &lt;?php print $comment_form; ?&gt;
  &lt;/div&gt;
&lt;?php endif; ?&gt;