The KWin View On Mutter In GNOME 3.0
His comments aren't anything too extraordinary, but they can be found here. Martin explains how Mutter uses Clutter for its scene graph rather than a custom OpenGL creation, how KWin is more like Compiz than Mutter, and how introducing a new compositor can be quite different.
It can be particularly difficult with a new compositor on Linux due to graphics driver issues being far too common. He also cites the tight integration between Mutter and the GNOME Shell (and Unity to Compiz) compared to KWin and KDE.