In my opinion, Xfce4 is great (and Xfwm4 in particular), but its most frequently visible component, Tabwin, has at least two uglinesses.
1) Top label with window class is uppercased. It's better not to see it at all, than see it screaming all the time you press Alt+Tab. The Gtk theme option to hide it could be great, if it's not possible to convert window class name to Title Case.
2) The container with icons has hardcoded old-fashioned inset border. Option to disable this border could be great, too.