I think that the last episode was first and foremost a nod to the hardcore fans. The show wasn't about the island. It was about the people. In the end, the Island was a backdrop to some interesting people put in an unusual circumstance, and it allowed some great human drama to be played out.
This was a show that mastered the backstory as a way of storytelling. You saw where they were, and those little backstories was what caught everyones attention. What made them LOST wasn't that they were stranded on the island. They were already LOST. The island became a metaphor (imo) for something that relatively few it seems are succesful at; the need for introspection. Being stranded allowed everyone to behave to either their lowest (Ben) or highest (Locke) denominator. It allowed them to be who they really were and to reflect on their past, to have a showdown with themselves, to look in the mirror and ask "Is that who I was? Is this who I am? Is this what I want the rest of my life to be about?"
The Island gave us all that we needed to fill in the blanks, myths and legends, good and evil, fear and doubt, faith vs logic. Love vs fear. (the love vs fear angle, which I admit is my rorshach test, was always very compelling to me, summed up best by Locke)
I kept up with it somewhat, but I lost track of LOST shortly after I left the cult. (only one set of myths to figure out at a time) I am going to rewatch from episode one. For me, the show was actually simple. I never got why people tried to "figure out" the nuances, like the island, and all the bad guys and time morphs. Imo, it was all a device to tell simple human stories of LOST people. To me, the final episode validated that for me more then anything.