There is a lot unanswered, but I think the main, overriding lesson of the series is that we're all Lost. The entirety of the series has been Jack's "Hero's Journey". From the beginning, Jack was forced into the role of leader and savior to the survivors of Oceanic 815. He was always fighting that, living in his father's shadow. He had never, ever, said goodbye to his father. He lived in his shadow the entire series.
The last two episodes saw Jack realizing he had to finish his journey. He had promised to get them all off of the island. Though he didn't succeed (not in every way, but as much as he could), that final shot of him looking up and seeing the plane leaving, and knowing that everyone that was left that wanted off the island had finally, truly, escaped, was his point of acceptance.
He had come full circle.
The point that I knew what was happening was when he was bleeding in the same spot on his neck in both "realities". I also started to realize that what was happening to the survivors in the "alternate reality" (for lack of a better term, let's call it "purgatory"), equated pretty closely to some ideas presented in Plato's Republic. I knew they were all dead.
Light and Dark, the airplane, the "heart of the island"... it all made sense.
It didn't answer every question, but the Island never was the mystery to be unraveled. There never was a mystery, just "shadows on the wall" that we, the viewers, were trying to decipher. It was the journey of one man towards acceptance, so that he could let go and move on.
Sometimes, you just have to sit back and watch. It's the journey that matters, not the destination, and it was a hell of a journey for me.