The specific way that John is different than his brothers is not clear. The reader is meant to infer what it is that makes John initially different. John's society has a certain set of rules. They are not allowed to go to the Dead Places, go east, touch the metal, or go to the place of the gods. The only people that are allowed to go to the Dead Places and touch metal are priests.
Because of these rules, there is a general fear of the Dead Places and the metal. Early in the story, John tells the reader that his father, a priest, took him to the Dead places and handed him some metal. John did not run in fear and did not die when he touched the metal.
"Then my father came out with the metal—good, strong piece. He looked at me with both eyes but I had not run away. He gave me the metal to hold—I took it and did not die. So he knew that I was truly his son and would be a priest in my time. That was when I was very young— nevertheless, my brothers would not have done it, though they are good hunters."
I suppose in one way John is braver than his brothers. The text seems to indicate that John's brothers would have run away or would have been too afraid to touch the metal. That's one difference. The other difference is a result of John's bravery. Because he didn't die after touching the metal, John is chosen to be a priest. John is held to a higher standard of rules, but he also gets to go to the Dead Places and collect metal. He also in entrusted with special knowledge and spells.
"I was taught the chants and the spells—l was taught how to stop the running of blood from a wound and many secrets. A priest must know many secrets—that was what my father said."
Lastly, John is different from his brothers, because he was taught to read.
"I was taught how to read in the old books and how to make the old writings—that was hard and took a long time. My knowledge made me happy—it was like a fire in my heart."