What is significant about how the animals arrange themselves as they gather to hear Old Major?
Other points of significance include:
Moses is markedly absent. Moses (as the name suggests and as you discover later in hearing about a place called SugarCandy Mountain) represents religion. Faith has been removed from this set of ideas about to be shared.
Benjamin the donkey is introduced as old and cynical and enters as one of the last. This is important because of what he represents. He has likely seen ideas come and go and get repeated in life, they are just pitched differently. So we find him in the back of the audience with the other horses, or the working class.
Those closer to the front of a class usually get more information than those in the back and easily become believers in the teacher's words. Those in the back just take it and survive... keep that in mind throughout the book as you watch these guys.
I see a few things as significant, given what we know will happen in the rest of the story.
First, look at who takes up the front rows. It is the dogs and the pigs. Later on, we will see that these two kinds of animals will push their way to the front. The pigs will be the ruling class, while the dogs will be used as their "muscle." The dogs will do things like killing other animals at the order of the pigs.
The other thing I see is how Boxer and Clover behave. As will be the case later on, they are the ones who seem to care most about the other animals. They try to help those who are weak.