This is one of those books that tries to ask the question of why the European civilisation managed to succeed where all other world civilisations failed. His conclusions are interesting, however we must remember that even now, fifteen years after this book was completed we are seeing an economic collapse in the west against an economic resurgence in China, India, and Brazil.
One of the ideas that he had was due to the axis of the continents. The only continent on a horizontal axis is the Eurasian mega-continent, which means that the entire continent is located in the temperate zone. However I find that idea to be a little dubious. Okay, granted, major civilisations arose across the continent: Europe, Arabia, India, and China; and trade existed among these four civilisations, however it does not really account for the rise of Europe to the exclusion of the rest.
One interesting idea I would suggest is to do with the Mongol hordes that smashed across Asia and the Middle East. In a way the Middle East never recovered from those invasions, and China was knocked back hundreds of years. Europe, by pure chance, managed to be spared, which meant that they were able to continue to develop ahead of the other Eurasian powers. This also resulted in them developing shipping which enabled them to take control of the Arabian Sea from the Arabs (since the land routes had been subsequently blocked by the Mongols).
The idea of trade along a horizontal axis is interesting, and we note that there was not much trade between the civilisations of America. Even then simple trade is unlikely to be hindered by tropical conditions. One thing about the American tribes is that they never developed shipping, which would have been something that prevented them from trading over long distances. In Africa we don't see much in the way of trade either, and the closer you get to the equator, the less developed the nations you encounter.
The concept of germs is interesting in that disease is a barrier to development. With malaria in the tropics it makes development of civilisation there difficult at best, and this can also prove a hindrance to trade, and Malaria is a disease that does not thrive well in colder climates. We also note that the common cold wrecked havoc in the Americas, where as the only disease we inherited from the New World was chlamydia, which is a sexually transmitted disease (the common cold is airbourne). However, that does not mean that Europe had been spared its devastating diseases, as the black plague ran rampant for over five hundred years and accounted for the death of up to a third of the population at its height.
Guns and steel deal more with technological development. The Eurasian civilisations had steel and guns were imported from China, but some suggest that the Chinese never used gunpower for weapons. That is rubbish. The Chinese had developed and were using explosives and rockets in warfare, but that did not save them from the Mongol Hordes. Even then, guns were a pretty poor weapon up until the eighteenth century, and were still quite inadequate. It wasn't until the 20th century that their truly devastating power comes into play.
The the balkanised nature of the European states also has a part to play. There is a point in that because there are all these competing countries all trying to gain an advantage over the other, which results in a perpetual arms race. That did not happen in China because the Chinese government pretty much controlled the entire region. However, that does not account for the fact that such balkanisation existed in the Americas and Africa. This also existed to an extent in the Middle East and India, but both of these regions, for a long time, have been under the dominant power of a single empire (the Ottomans in the Middle East and the Mogols, and later the British, in India).
To me, the only real answer to this is that not only was Europe spared the ravaging of the Mongol hordes, but that we Europeans are in reality a sadistic and savage lot who are always looking for faster and more sophisticated ways of killing each other. It is not that the Native Americans lived in peace, they didn't, it was just that they were satisfied with using basic weapons. Even when they did have access to guns, they took them and then turned them on each other.