this text is solely about a idea for a project. The KDE4 site  has given their reasoning on why to reject Compiz. Expectedly Gnome will jump in similiar wording. I just want to raise one point about compositing effects/plug-ins. Plug-ins form a great users space to interact with ideas and code.
It is correct stated that Compiz exposes too much details about its internals into plug-ins. So its hard to reuse compiz plug-in code for say kwin.
Anyway the underlying logic is pretty simple.
Shaders are small text files. The desktop has a visible and virtual size, workspaces, monitor outputs, windows, subwindows, edges, a task bar, shortcuts, a property system (Xorg Xatom’s) and some more. So from a logical point of view it could be possible to write one plug-in for serveral compositing engines.
What I, as a plug-in author, would find great is a project, which developes a common script language and implements native plug-ins for each of those window managers acting as a script host. Based on a simple and common logic script authors could write text only plug-ins. Obviously shaders are very likely to be part of such a system or possibly OpenCL if mature enough.
I know coming with code or a detailed proposal is nicer. But if someone searches for a nice Summer of Code project, that might become one.