According to Jared Diamond in Guns, Germs, and Steel, what role did the East-West axis play in the differences between societies?

The answer to this question is given in Chapter 10 of Guns, Germs, and Steel.  The length of a landmass’s east-west axis helps to determine how well domesticated plants and animals can spread across it.  This, in turn, determines how easy it is for civilization to spread on that landmass.  Places where civilization spread more easily became the “haves” of the world.
Diamond argues throughout the book that areas that have had agriculture the longest are the areas that became the “haves” of the world.  The length of the east-west axis helped determine which areas got agriculture.  Agriculture only arose independently in a very few places.  On p. 177, Diamond says that this only happened in “no more than nine areas of the globe, perhaps as few as five.”  If agriculture only arose in a few places, that means most places got agriculture by having it diffuse to them from places where it arose.
This is where the east-west axis comes in.  Plants and animals can spread more easily along an east-west axis than a north-south one.  Places on an east-west axis will typically have similar climates (allowing, of course, for things like elevation differences).  When places have similar climates, plants and animals that flourish in one can also flourish in the other.  If agriculture developed in a given place, the plants and animals that were domesticated there could spread much more easily in an east-west direction than in a north-south direction.  For this reason, agriculture spread across landmasses that have long east-west axes much more rapidly than it did across landmasses that were long from north to south.  This helped create a difference between societies in places like Eurasia (long east-west axis) and the Americas (short from east to west, long from north to south).  The places with long east-west axes got agriculture more easily once it arose, thus becoming civilized earlier and having a head start in terms of developing technology.