This was an interesting visualization. Nothing wrong with it as long as you understand what is being measured, but I think it does leave out degrees of density in very urban areas. Chicago and New York are both 100% urban, and that does indeed tell you something, but there's still an interesting measurement to take above the threshold used here for what qualifies as "urban".

I actually find this review more revealing:

http://beyonddc.com/?p=4808

These maps show distinctions at very high density levels, so you'd see not only how Chicago differs from New York, but distinctions within these cities at a very high level of density.