These were my thoughts on the Lost finale (I posted them on another forum a few months ago):
The battle between good and evil, and faith and science, that occurred on the island was a parallel of everyday life. Through their experiences, the characters were able to have faith in themselves, faith in each other and possibly find their soul mate. They were given a purpose and learned to protect the light and keep the darkness at bay. In the end they became altruistic, with several characters risking their lives for the sake of others. Their life on the island was the only time they felt complete and this is what connected them to their afterlife.
Some people were confused by the ending and think that the island is not real -- but it is. The flash-sideways was somewhat of a purgatory in which they were waiting to be awakened and made aware of their previous life. Once they had all been awakened, they were able to "move on".