High quality, clear delineation of detail--I hate when it's hard to tell from a map what's cover, what's solid and what's passable, and this is very evocative while remaining discernible.
For virtual tabletop usage, it's always irrititating to have to manually count out the grid squares in the gridded version in order to implement the gridless version at the right size. I wish the product description had the exact grid dimensions (23 by 16).
Also, there is a margin around the egdge that contains art but is not intended to be part of the grid. While this makes perfect sense for printing, it always makes life difficult when implementing in a VTT.