If you take a critical look at all of the castaways, they are all damaged, tormented people that have done bad things that they deeply regret.
Sun - Cheated on her husband, lied Jin - Lied about his father, worked as muscle for Sun's father Sayid - Torturer Shannon - Greedy user of people Boone - In love with his sister Sawyer - Con man Kate - Murdered father, caused the death of Tom. Charlie - Junkie Hurley - Stole winning lotery numbers from a mental patient Analucia - Murderer Eko - Killer, druglord Desmond - Coward Claire - Almost killed her mother Michael - Poor relationship with his son, feels extreme guilt, is a victim, and it drives him insane. Locke - Obsessed about being taken advantage of, costs him the woman he loves, criples him.
And Jack - Turned his father in for killing a patient while under the influence. Spurned an affair and remained loyal to his wife, only to find out that his wife was cheating. Is obsessed withh 'fixing' people.
All of the castaways are broken, and feel the exact same pain from the things they have done wrong. Jack's pain however, is caused by the things he did right, and it haunts him more so than the others are haunted by their misdeeds.
Jack has always been the leader. It took the influence of Locke to make Jack 'complete', and next season will be Jack Shepard's.