I'm still trying to decide if each of Kay's books focuses on a single theme, but this one definitely does: Under Heaven is all about power. Wealth, influence, physical strength, knowledge, family connections - all these and more run through Under Heaven, exploring how individual lives and the fate of nations are shaped by the power that people have over each other, and the ways they choose to use that power.
Kay's worldbuilding, as usual, is excellent - there's enough scenery to catch the imagination, without drowning you in detail, or telling the reader things just to prove that the author knows them. Characters are well-drawn, with a believable shared worldview, and extremely human. They might be more or less sympathetic, but they mainly avoid being caricatures.
On the topic of characters, however - there are just too many, and some of them are too random. I can see why you might want to narrate a scene from the point of view of the guy on top of the wall. I guess. But do I really need his backstory? In my opinion, I do not. In my opinion, it interferes with the story. Also because of the character-jumping, I had some trouble keeping track of who-was-supposed-to-know-what.
Overall, I enjoyed Under Heaven, but I didn't respond to it whole-heartedly. Perhaps that's because I don't care enough about the subtleties of power, or perhaps it's because this is one of the less fantastical of Kay's books (I like when there's lots of magic in my stories.) In any case: I cared what happened, but...not as much as I often do, with Kay's work.
I'm currently debating whether to go back and re-read a bunch of Kay's other work, to look for these recurring themes. It came to me, very clearly, that Tigana is about loyalty, and that I REALLY responded to that book. Off the top of my head, I would say that Fionavar Tapestry is about identity, the Sarantine Mosaic is about legacy and creation (what we leave behind us), and that Lions of Al-Rassan is probably about courage. Thoughts?