Aside from the easy use of the map, I believe that from an amateur map makers stand point it is a well constructed map scheme. Personally i found the pop density shown as different shades of a single color to be much more helpful than several different colors all in one map. Also I think that brown is a good color to do this with because it is easy to distinguish multiple shades.
I also like the setup of the legend because it doesn't use the traditional style of putting its tittle at the top. The reason I like this is because the user can still tell what the map is about because the tittle is the first thing that the user looks at because of its size, and then I can organize what information Id like to see before seeing the results of the current map.