Here's my little theory about the Lost island.
The island was real, with special powers. It could heal people. And not just the physical wounds like Locke's legs or Rose's cancer. But it had to heal the whole person once he or she landed on the island. We kept hearing that the island wasn't done with them yet. I think it meant they weren't healed.
Each lostie was hand selected by Jacob because they were broken. The real gift Jacob gave them was the chance to land on this mystical island which could heal them inside and out. The middle seasons were not just filler. Actions happened that brought the broken souls closer to being whole. As Locke told Charlie in the cocoon episode, struggle is nature's way of making you stronger.
Here are just a few:
Kate was broken because of the family life...the father she loved turned out not to be hers. Her blood father was evil and she killed him to protect her mom, and, as she told sawyer, because with him around she could never feel or have any good in her life. But her mom betrayed her and turned her in. Kate was always climbing or nearly falling out of trees. At first I thought it was a clue, but later realized it was symbolism for how strongly she needed family roots that were denied her. Events on the island lead her to deliver Claire's baby, take him off island to protect him, then risk everything to reunite mother and child. Something she never got in her own life. This process cured what was broken in her. In one of the last scenes you see her sitting on a huge uprooted tree, as if she has conquered it. Aaron was never meant to be a big mystery in the plot. He was put in the story to help Kate resolve her own issues about being separated with her mother.
Sawyer was broken because he saw his parents die in front of him. It made him never trust or get too close to people. Sawyer had no use for people unless it was to con them. But events on the island made him learn to love the losties. One interesting event was when Locke snatched Sawyer, fresh out of bed with no shoes on, (he was going to the bathroom after Kate left his tent) to go into the Black Rock and kill the con man he was looking for all his life. It was just like he took little James, hiding barefoot under his bed when the killing of his parents happened, and gave him the chance at revenge he needed to move on. By the end, Sawyer was running through the jungle saving everyone. Even jumping out of the helicopter to lighten the load for the other's trip home. Events on the island made him care about people again, which cured him.
Jack is easy. He feels like he never measured up to his dad. All his life he was trying to fix things. If he could fix something really big, maybe he could be cured. He has to save the world to save himself.
I also think a lot of the events we thought were clues were really the writers using symbolism. Jack wakes up in the jungle in the pilot episode next to a little black flashlight. At first I thought it had to mean he time traveled. But in a couple episodes later, he is shining the little light into the fallen plane when the boars were in there and Sawyer comes up behind him and flashes a huge flashlight over his shoulder. That's symbolism. Jack never felt he measured up. Kate is aided by a black horse while on the run. Later she sees it in the jungle and finally she is able to approach it and touch it after confessing to Sawyer why she killed her dad. The black horse symbolized she was about to go on a dark ride or journey. Locke becoming the smoke monster was perfect as both he and MIB felt overwhelmingly trapped and frustrated. Locke in a wheel chair and MIB on the island. A lot of these things weren't clues. Just good writing.
Now that the show is over it's fun to go back and try to make out the symbolism and the different ways the losties were cured by the island. The ending was perfect. Once they were all cured, the island was done with them. Jack was the last one. As soon as he took his island medicine and saved the whole friggin' world, he was finally good enough. He could move on. And they chose to go together. What a tear jerker.