I thought Diamond's comparison of Europe and China was especially weak. He wrote it off as that China had an emperor, and thus a single point of failure. He didn't do enough to explain why Europe did not become an empire, or to explain why China's immense bureaucracy did not avoid such a single point of failure.
I remembered he did explain why Europe failed to become a united empire - geography.
According to the book, the reason China has a large stable empire was because of China's geography, which was mostly flat and well-connected by the Yangtze and the Yellow river. With few geographical segments, this led to the people being less likely to form 'tribes' and more likely to be united culturally.
This is in contrast to Europe, which has a large impassible mountain range in the middle, and lots of peninsulas and small rivers, forming natural boundaries. This led to Europe being more diverse culturally.
