Just some feedback about the overall artwork and look.
1. I strongly suggest you use a patched freetype and fontconfig as default with sub-pixel rendering (slight hinting/rgb). The default fonts in any GNU/Linux just doesn't look good at all without a patch. Personally I prefer Infinality, but Ubuntu's patches are fine too. With infinality you can use a similar look to Windows cleartype.
2. As I understand you will probably rely on contributors for artwork. Have a guideline
. For example, try not to have a logotype in the wallpaper. Use a smooth non-distracting default wallpaper. (See Ubuntu and OSX default wallpapers for inspiration.) Branding with a logotype can be an icon on the panel's so called "start-button" or "menu-button". Same with the default theme. Non-distraction themed themes. Look for something good on xfce-look.org with the right license. Make sure it doesn't have anything that screams "here i am" unless it's for something useful. Also make sure the theme works fine in both gtk2 and gtk3 applications (since xfce is now somewhat becoming a gtk2-gtk3 hybrid.)
And as for the desktop switcher in the panel. Are you sure it's needed? I think a Windows user who doesn't know much about interfaces can be scared if they accidentally click on it. They'll probably wonder where all their open applications went.
I also believe it's a sore part on the overall look.