The biggest problem with this book is what's not present. The bulk of the book is short chapters describing each of the countries in Tian Xia, in alphabetical order. Each country has a description of history, geography (which is very broad strokes and mostly about the climate), daily life (food and schedules), yearly festivals, political figures, and key locations (which are nearly all brief descriptions of big cities). However, the countries in the first half of the book have barely anything in the way of adventure hooks. Starting with Minkai, we start getting more details about potential threats, but that makes half of the countries very challenging to incorporate into a game. The countries that do have adventure hooks are still very light on details.
It feels like the developers spent so much time trying to break down stereotypes and portray Asian-inspired countries in a positive light that they forgot that an RPG requires conflict. As a GM, if I don't have anything for my PCs to fight or investigate, it doesn't matter if people in the north of a country make their morning porridge with fruit or meatballs. In practice, I'm also more likely to insert regional holidays when I need them for the plot.
A few other more minor but significant omissions. It looks like Paizo only paid for one map for the entire book - there's no map of the territory held by ancient empires, which was a very useful addition in the Inner Sea World Guide, and the maps for the individual countries also don't add any more detail beyond the big map and don't show the names of neighboring nations. Maybe a central explanation of what languages are available to learn and where they are spoken was moved to the Tian Xia Character Guide, but there's no mention of that book anywhere in this one. Hopefully a more thorough discussion of ancestries and powerful foes was also moved to that book. Finally, a list of additional sources to use for inspiration would have been nice.
If we were going to get as much support for Tian Xia as we do for Avistan, these omissions would be fine. With only two sourcebooks and one AP every decade, though, the sourcebook we do get doesn't give Tian Xia the detail it deserves.
A good campaign setting book, best enjoyed in small sips every now and then.
Things I liked about this book:
It's a great campaign setting source book. It manages a good balance between familiar and exotic, and it has a more grounded approach that more or less succeeds at avoiding "east asian fantasy" stereotypes (to give a silly example, ninjas are not mentioned even once).
The countries and lands presented therein are quite distinct from each other, and the book really shines on putting this uniqueness in the forefront: I really do like how the first subtitle of each section really conveys the feel of each land.
A minor detail I liked it's that the book is mostly the setting itself. There is very little in terms of mechanical content, which for people who like playing other games and just adapting stuff, is just great.
Things I don't like:
I know this might be just me, but the book itself is poorly designed to be read "continously". Some parts can get quite repetitive (after all, there is not a lot of ways you can say "people get up early to work" in each of the "A day in..." sections), and... I get the point, so that people can just extract only what they need, but the lack of self-reference and the incredible internal consistency makes an otherwise flavorful setting into a collection of things that feel same-y, even when they are not.
To give a spin on an otherwise cliche-y phrase, I like this book as I like my liquors: having a sip of it every now and then, alternating with other stuff in between. I would have probably enjoyed it more if I read a chapter a month, so to speak.