I think that the dominant theme in Terry Pratchett's writing is systems of organization, whether they be political, scientific, religious, or magical, and how they interact with the messy granularity of individual life. Even in his earliest Discworld novels, before they become social satires rather than genre parody, he comes across like a kid showing off the neat, clever model that he built.
This is why I think his novels are so popular amongst socialists, even though he kind of pooh-poohs the idea of revolution and seems to think that a civic-minded aristocratic tyrant would be the best at running a government; because his focus tends to be sociological, at least as much as psychological.