Immediately following my first watch of the Lost finale, my thought was what? They're all dead? Have they been dead the whole time? Then I re-watched and came to the conclusion that the island was real and everything that happened to them on the island, flashbacks, and flash forwards were all real, they were alive, and it all really happened. In the flash-sideways, they were dead and found each other so they could move on together.
Case closed, right? Wrong. There are a lot of people who believe the Losties were dead while they were on the island. As you can see by the comments in the discussion of the island's powers, there are a few people who believe that the island story was the afterlife of the main characters.
I still firmly believe that everything that happened on the island was real and the afterlife was the flash-sideways. What do you think? Discuss in the comments.