[Raphael]

I am fascinated with the idea of customizable window managers. I haven't delved that deep, but the difference between XFCE and Unity is night and day. I can choose color schemes, window decorations, and make custom task bars. This should be table stakes for graphical interfaces.

[muyuu]

Have you checked Enlightenment lately? it's a decent shot but they seem short on workforce.

Also http://awesome.naquadah.org/ and https://code.google.com/p/wmii/

And that's just windows management which is probably the most obvious advantage of an all-open approach, but there are other things like user programmed priority systems, memory management etc which could potentially do a much more effective usage of the hardware than the standard monolithic opaque approach.