Display comment titles only
Drupal 7 does not have this feature. 

All you have to do is create your own javascript/jquery (.js) file and do something along these lines. 

(function ($) {
     $(".commentbody").hide();
          $(".commenttitle").click(function() {
          $(this).parent().find(".commentbody").slideToggle();
          });

})(jQuery);


Here is an example JS FIDDLE