This is a concept I've been working on.
Here are some guidelines I followed while making this:
No custom software. It has to use an established GTK engine. This one is made with the Aurora GTK engine in mind, which sports a very polished look.
Few custom icons. It was made with the Tango iconset in mind.
It's visually neutral. The window decorations are not orange or brown — they’re predominantly dark gray. This is good so that when you’re viewing images or working on visual things, the colors of the decorations don’t influence your perception of the content. I think this is a strong point of OSX. Let the wallpaper define the look, not the window controls.
It's dark, to be similar to the current Ibex theme.
It uses the default two-panel configuration of GNOME. Not only is it proven functional, but people have been used to this and it would be awful to break their expectations. No AWN docks or anything fancy.
Anchor(ConceptArt) Concept Art