Gravatar Hovercards (Jetpack) jQuery error

You are loading jQuery 1.6.3. .on was included in jQuery 1.7 (I believe). jQuery itself is up to 1.9. You need to be using a more recent version of that library– basically, just let WordPress load its own version and you should be fine. It looks like most of your scripts are loading from your domain except that one, which you are loading from Google’s libraries. You aren’t gaining much if anything by doing that and more and more scripts will break if you continue to use that old jQuery. If you want to load to form Google’s libraries you need to pay attention and keep up to date, or use one of the several plugins that keep track for you.

Edit:

Change that callback function to match this:

function jcp_jquery() {
    if ( comments_open() && ( is_single() || is_page() ) ) {
        // wp_deregister_script('jquery');
        // wp_register_script('jquery', ("http://ajax.googleapis.com/ajax/libs/jquery/1.6.3/jquery.min.js"), false, '1.6.3');
        wp_enqueue_script('jquery');
    }
}

If the plugin still works you are done. If the plugin does not work then it is probably dependent upon that old version of jQuery and will have to be rewritten to be compatible with the newer jQuery.