According to Guns, Germs, and Steel, what was it about the Americas that put these two continents at a developmental disadvantage?
The Americas suffered from two main disadvantages.
First, there were not very many domesticable plants and animals in the Americas when compared to Eurasia. Many of the animals that might have been domesticated were killed off when humans entered the continents. The plants simply were not as "good" as those in Eurasia.
Second, what domesticable plants and animals they were had a hard time diffusing due to geography. The Americas have a long north-south axis that makes diffusion difficult. In addition, there are many natural barriers like the jungles at the Isthmus of Panama and the deserts of Northern Mexico. These barriers made it so that breakthroughs could not be shared easily between societies and each society had to essentially invent everything for itself.