Although there are still dozens of unresolved questions and whatnot, my take on the finale was that it reminded us that the show was clearly far more about the human condition and the characters growing and changing than merely a sequence of events and unraveling mysteries.
Each character came to the island with a troubled past and something they needed to "let go" of. These flaws are a prerequisite for each character to become a game piece in the little game that Jacob and MIB are playing.
To let go of your past, it requires that you make a choice to do so. Each character's experiences on the island helped them to do this, but they still had to come to the realization on their own that they could change, or that their past could be overcome.
This is why, in my opinion, the real conflict was not MIB trying to leave the island, or Jacob needing a new protector/replacement. Those things were just catalysts to force the characters to continue to make these choices, to rise above their pasts and their flaws to do the right thing in the end. Jack's sacrifice, Hugo's acceptance, Ben's road to redemption (although possibly incomplete based on his chat with Locke outside the church), Kate's admission that she truly loves Jack and wishes for them to stay together, all of these things actually happened in "real life".
The question is why do the characters have to repeat their struggle to deal with these issues after they've died? I think it's because, even at the time of their deaths, they still have not understood (or remembered) enough to move on. They all have what they THINK they really want out of life, but even in the afterlife, there is still a missing piece of the puzzle that they either can't identify or are completely hung up on.
That missing piece is their respective experiences on the Island. That's why they were brought there. Not to accomplish any grand plan of world salvation, not to find out what was up with the DHARMA Initiative, not to push a button. Just to BE THERE, to be involved in experiences that would teach them that they could still grow, change, and get over whatever they had left behind.
Jacob brought them to the island to prove his point about human nature. Not to save the world, or protect the island. The actual events of the island are completely irrelevant, which is why in the FST, it is at the bottom of the ocean. The lesson has already been taught, and by that point, school's out forever. In the end, it was up to each character to either apply the lessons the Island taught them and find enlightenment, or not. If they could do so, they could move on.
The Island is Locke's Walkabout applied to every single character, just on a much grander scale.