Non-color and non-glossy pages. If it has color and glossy pages, that's two strikes against it.
See, and for me it's the opposite. My all-time favorite RPG book, for appearances sake (though the fact that I love the system and setting surly don't hurt any) is the Star Wars 2nd Edition Revised & Expanded RPG book. It is full-color and glossy all the way through baby! My second favorite (even though I really don't like the rules) is the D&D 3.5x Monster Manual (the 1st!), and it too is glossy and full-color.
Last is maps. I want a legible map with some areas almost devoid of settlements. I want room to add my own stuff to the campaign setting.
Yup, yup! Maps, maps, and more maps. Maps of all kinds - preferrably full-color and not cartoonish/hex-based.
Flavor, mucho flavor! For example: To read, my favorite book is
The Wanderer's Journal for Dark Sun. ("I live in a world of Fire and Sand." *shiver* Awesome.) It was almost entirely flavor, and the only "rules" stuff were the stats for some of the more common, Dark Sun specific creatures in the back of the book.
From the criteria you set above, that it is a setting book and not a rules addendum, I would go with the following criteria, in order:
1.
Flavor - The setting itself has to grab me. But as this is very subjective, even having it on the list is a little silly. but it is that important. Even knowing that everyone knows it is important, it is important enough to still stress.
2.
Images (Art and Maps) - Art can convey as much setting information as the writing, sometimes more. Let's use Dark Sun as an example (again): The Brom art (moreso than the Baxa, in my opinion) really gives you a feel for the setting, making sure you understand that it is not your typical fantasy setting.
3.
Maps - I love maps. Even if I don't use the setting, I will often grab it if it has good maps, like I did with some of the Accordlands stuff. Large world maps, local area maps, specific structure maps, town maps, city maps, village maps, they all are needed to help flesh out the setting, because
architecture can really express a tone, and it is quadrubly effective if you also have images of what the structures look like from the outside.
4. Creatures/NPCs - It is nice if a setting has some setting-specific creatures, and some examples of NPCs really help a GM learn what to look for when making their own.