I think many maps use some common principles such as color, consistency and iconic representation that supports depth of processing through a deep exposure to rich details.
This is a map of Downtown Portland, OR and is interesting to me because of the caricature like iconic representation and bright colors to communicate points of interest and landmarks. There is s a lot of consistency across the iconic representations used here which is expected and lends to its overall readability.
Flexibility-Usability Tradeoff is at play here. While these icons are fun and the map has a lighthearted feel to it while providing basic information, there is a bit of tradeoff occurring as some points of interest are too dense based upon this maps point of view. This leaves some streets unidentified for a short period while more important details override. This leaves the user to determine and track the street. Because of the nature of this map, this may or may not be acceptable.
Highlighting is seen where information such as street names are presented, but other items such as building names, landmarks and points of interest are all in bold or even in a color with a graphic.
Horror Vacui seems to be at play here and I think it is one of things that makes the map interesting. When thinking about the type of map I wanted to use for this exercise, this is much like what I envisioned. Typically, I would say Horror Vacui has its place, with this design and the lighthearted feel you get from it, and considering it is a map, I think more can be better to a point and so the principle is useful. I think the best way to see this principle at play is by looking at the entire image.