Having a separate GUI Domain is going to be a big win for Qubes IMO. When testing out on my own devices, it was practically impossible to make UI changes (e.g. installing a menu-bar, switching to a different WM) without significant risks to dom0.
I'm hoping this will make it much more approachable for less "hardcore" users since it can be made to look good, without having to worry about the risks that come from complex UIs.