You actually beat me to doing mockups, which I was going to try to do when I had some time. I dunno how feasible it would be, because I've not looked at the YaST2 codebase, but it would be very cool if YaST2-GTK+ was HIG-compliant as far as possible. My vision for the GTK+ frontend is that it allows YaST2 to appear more integrated with GNOME (and XFce) and the HIG will help that of course.