One quality that makes Ralph a good leader is that he (mostly) seems to care about the kids in his care. By the beginning of chapter 2, he has explored the island and is able to begin formulating plans for survival. He realizes that there are no other people and that he is going to have to establish some sense of order so that the boys can both be rescued and survive until they do. Thus, it is Ralph who makes sound decisions for the group, such as deciding they need a signal fire. It is Ralph who looks around at the feces everywhere and tells the boys that they cannot use the entire island as an open restroom. It is Ralph who understands the importance of shelter and continually works to build crude structures for the group. Ralph has the survival of the group in the forefront of his decisions and actions.
He also shows bravery, which is necessary in good leaders. When Jack lets the signal fire go out and they miss the opportunity to be spotted by a passing ship, Ralph does not shrink from confronting him. Ralph shows courage when hunting for the beast in the jungle. He doesn't allow Jack and his hunters to tackle the job alone; he insists on going himself.
Ralph also develops a sense of wanting to keep the group together. He resists Jack's efforts to break apart the group and rallies to keep the littluns safe and close by. Ralph sees value in the complete group, even when he notices that more and more of the boys trickle to Jack's camp.
Ralph is far from a perfect leader, especially in the beginning of the book. But he grows with experience, learning from mistakes along the way, which is another quality of good leaders.