From what I understand, it's his humility and compassion. Even though he's essentially the leader of the Catholic church, Francis doesn't flaunt his power. He even refuses many of the services offered to him (for example, he'll ride a public bus instead of the bulletproof popemobile). Additionally, Francis preaches love and kindness, even to those who believe differently than the church. His views represent an acceptance of all people, including sinners, rather than a judgment of them.