I think the most important characteristic of Equality 7-2521 is curiosity. The entire story is driven by his curiosity and it is what saves him from the World Society at the end of the book. I also think that his curiosity is what will eventually save the rest of the population from the society. If one person can escape as easily as Equality 7-2521 did, then others may figure out that they need more in their lives and be able to escape as well.
Equality 7-2521's curiosity is illustrated at first by his discovery of the tunnel with International 4-8818. However, once the discovery is made, he continues to go to the tunnel in an effort to learn more and more about what is down there. Eventually, his curiosity gets him temporarily into trouble when he loses track of time and neglects to come back to the Home of the Street Sweepers by his curfew.
Importantly, Equality 7-2521 is unable to stop being curious. He is driven to learn as much as he can and he sees it as a gift, as illustrated at the end of the book. He has found books to read and he is elated:
It was when I read the first of the books I found in my house that I saw the word "I."... I understood the blessed thing which I had called my curse. I understood why the best in me had been my sins and my transgressions; and why I had never felt guilt for my sins.
Equality 7-2521's sins and transgressions all stemmed from his curiosity. They were his curiosities. Having realized this, he is able to freely explore his curiosities for the good of mankind.