Having read Dante's Divine Comedy way back in the day, I can tell you that I'm firmly of the belief that this whole thing is the writer's interpretation of Dante, especially Purgatory. According to Dante, Purgatory is where those who are in limbo as to their final resting place go to experience a final "test" of what kind of person they are and where there final destination will be. It takes as long as it takes and everyone can experience it differently if they must. This is the island. All the strange situations it presented to the characters are the tests. This is why they are somewhat odd or unique. Here's the thing I got from the show as a whole: everyone died in the crash, in the very beginning. The island they experienced after, was their purgatory, and the choices they made as they were there defined where they would end up. I have many reasons to think this, but a couple are the very last scene in the last episode, they showed the wreck on a deserted beach. That's because everyone was dead from the beginning, and that's what it really looked like. Also, other things, like Linus not going into the church at the end. His purgatory was not complete. Notice how he asked for forgiveness, and when he got it, he said it would help. That's part of his journey through purgatory. The flash sideways is part of the acceptance of those who were deemed worthy to pass onto Paradise, and their acceptance of this final destination. There are tons of things that go along with this, but like I said, too much to explain here. I.E. the light at the bottom of the cavern - it's a passage to the Inferno that's sealed up, and it's men's greed that brings them there. All makes sense, and that's why it gets all red and angry when it's opened for a bit.