Material Layering Systems in Unreal; What are they? And why you should use them?

I've been working as part of the Arrowhead initiative to create a series of general art plugins to enable new projects to hit the ground running, especially if they are starting from scratch. And I've implemented a new shader system which breaks away from the old Ubershader method. The MLS (Material Layering System) has many advantages over older methods for both performance, quality and artist flexibility. And my current project's art team swears by the MLS now as the new standard for art production in games, with several other projects having requested to use my MLS too, so I thought now would probably be as good a time as any to do a talk on it.