Currently Metacity appears to be built with a composite manager enabled by default. Because Metacity's composite manager is buggy, it would be nice if there were a USE flag that toggled this feature. I've needed to roll my Metacity back to 2.8.0 (composite manager disabled by default in that version) and mask later versions to keep my desktop usable.
--disable-compositor is set in the metacity-2.8.{1,5} ebuild