> Metacity needs a new, stable compositor, that can use xrender or opengl depending on hardware/driver capabilities.
You have a lot of points that are valid if they matter to you. It's your time and I would be happy seeing you doing something that would make you happy to do. Seriously.
But this is a red herring. "2D acceleration" no longer exists for modern hardware, or at least any hardware going forward. You'll probably see it on embedded systems or Via's crap, but all that is going to go away also. One way or the other. This means that it only requires marginally more work to get basic OpenGL support then fast XRender support. And OpenGL is more important then XRender as on modern CPUs the 2D performance is just fine on pure software.
This means your choice isn't going to be between 2D or 3D acceleration... it's just a choice between having acceleration or not at all. Compositing is going to suck no matter what if you don't have decent acceleration. I think a Xrender-based compositor has VERY limited utility.
But you can do what you want. I just don't want you to put a lot of effort into something and you end up with the last time somebody put a lot of work behind a serious effort into (semi-)forking Gnome.
I recently built a new machine using All-AMD chipsets just because I appreciate their efforts. AMD CPU, AMD mainboard chipset, and ATI 5770-based video card. I was afraid I was making a huge mistake, but with a rather insane setup using xorg-edgers PPA (beta Mesa + New X + beta ATI drivers) on Ubuntu 10.10 I am very surprised by the quality of the experience. I'm even running 2.6.38 kernel.
I am using the Gallium drivers and despite throwing random games at it running compiz, flash video, (simultaneously) and trying different things that normally caused issues for me in the past in Linux with OSS drivers.... it's been dead stable. It's amazing and I doubt other people will have similar experiences, but for me it's working fantastically.