The major strength of this book is that it uses a great deal of scientific evidence to set the foundation for its social argument.
Diamond brings out a great deal of scientific evidence in the book. He looks at the distribution of grasses with large seeds. He looks at the distribution of large animals. He looks at the topography of various continents. All of this is used to build a very good foundation that shows that certain areas of the world were more conducive than others to starting civilizations.
Once he has created this foundation, he makes the argument that these natural advantages caused the Europeans to become stronger than the people of the Americas, Australia, or Africa.
The major strength of the book is that it sets up such a solid scientific foundation as a base for its more speculative social conclusions.