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Compiz is an excellent WM in it's own right, and a lot of the code for compiz is from Metacity, just modularized. Having used compiz and looked through the source, it appears that compiz is cleaner and more powerful than metacity.
With compiz, you are able to dynamically load and unload shared moudles which governs the effects of the compositing manager. The settings that these modules uses are grabbed by a configuration backend, the one currently in use is gconf.