I think some of the reviewers here didn't read the book closely enough to understand the context of some of Diamond's arguments. He never says that biogeographical effects are the ONLY causes history. His main purpose is the search for the ultimate, extremely general causes for the broadest of trends in human history and prehistory.
By the time the Mongols roared across Asia, or the Moguls invaded India, many cultures around the world already changed so much that bioregional factors, though seminal in the creation of these broadest trends, weren't nearly as important as the political, religious and economic ones. He is not ignoring religion and so on but, he states plainly several times that isn't his focus. He is looking for ultimate causes--before humans had extremely advanced mental concepts like religion.
He also wanted to point out the devastating influence of disease on history. It was surely the European germs that did most of the conquering of Native Americans. The guns and horses were almost incidental. Later on, once Europeans had established themselves, then we can focus on economic and political systems. But we can't ignore the effects of the diseases unleashed on the Americas. These plagues gave the Europeans a very lucky boost that catapulted them beyond the wealth and power of China, India or the Middle East--long before the Industrial Revolution made this gap obvious.
Another thing that some people seem to be having trouble with is his assertions about the native intelligence of tribal peoples around the world. (If you read the book, you notice that he is not just saying this about the New Guineans.)
He takes pains to point out what he means by this. He not talking about some mysterious genetic superiority of tribal peoples. It's all straight up culture. Tribal culture forces people to be better generalists than they'd have to be in literate civilizations. They can't rely on embedded support structures like books for memory or experts for obscure fields. They have to be pretty good at a lot things. Otherwise they die. They have to be better at memorizing things because they can't count on computers or books to remember things for them. Living in a dangerous, wild environment makes them cautious and aware of all that is going on around them. That was all he meant. The circumstance of tribal peoples force them, only in very broad ways and only on an individual basis, to be smarter and more curious than civilized people.
And in the end it does them no good. Because civilized societies are SMARTER than tribal societies. That is why tribal society has been steadily disappearing over the millenia. They just can't compete.
Finally, of course the book is repetitive. In fact he sums up his argument in the preface of the book. You needn't even read the rest if you don't want to. The rest of the book consists of him reiterating his points from different angles to point out the objections he has managed to answer and the many questions that still remain. He is just following scholarly practice and exposition--just to make things clear that he has thought about this.
He knows that his theory can't explain everything. In the epilog he points out that China, India and the Middle East are good counter examples to his idea. They each had an expansionist rise to great power--a time when they were unafraid to try new ideas and explore new ways of doing things. If the highly complex forces of economics, politics, religion had arrayed themselves differently. We might all be speaking Arabic now. Or Cantonese. Europe was just lucky to be in the right place at the right time for things to come together as they did.