Here is my immidiate reaction on that mockup gui..
* Very pretty
* Too Control Panel'ish like on the Palette Preferences. The Tabs are just glowing look-at-windows-and-draw-amiga-version.
* Too Win Explorer'ish on the Amiga Browser... Although I admit win32's explorer is quite handy. The button bar is just waste of screen space though.
* Too much details in the scroll bars. Simplicity is gone in your scrollbars.. The dark grey doesn't fit in.
* Too much theme-like in the windows titlebars. (Gradients, text outlines)
* Too much stuff in the titlebar - but I suppose you wanted to just show what should be possible..
Overall, it's very close to my vision.. But it does look a little like a theme and not a default look.. :-)
It's very nice - I am amazed I missed this one...
Keep up the good work and send it in to everyone (A inc, Hyperion)... If you make a descriptive text why a certain thing would be done this way and that way I'm sure you will make one or another designer come up with some idea even remotely related and by that you have make your contribution to the next GUI.
Peter