The one thing that I'm missing in 3.8 is the menu separators. While I love the new design overall, I personally don't like having everything mashed together with no separator lines between sections (it feels messy when plugins/themes start adding menu items) so I did the following to bring them back:

In my themes functions.php I added:

function wptutsplus_admin_styles() {
wp_register_style( 'wptuts_admin_stylesheet', get_stylesheet_directory_uri() . '/css/admin-styles.css' );
wp_enqueue_style( 'wptuts_admin_stylesheet' );
}
add_action( 'admin_enqueue_scripts', 'wptutsplus_admin_styles' );

I then created a new stylesheet called admin-styles.css with the following CSS (placed it in a CSS folder in my child theme):

#adminmenu li.wp-menu-separator {
background-color: rgba(255,255,255,0.15);
height: 1px;
margin: 0;
}