I really loved all of the emotional and touching "realization" moments in the episode, and the chance to see all of these beloved characters together again. But the last few minutes left me a bit cold. I was waiting all season for the explanation as to the connection between the two timelines, and the ultimate answer was vague enough that I don't believe there was a direct connection. Why was the Island seen at the bottom of the ocean? I have no idea, if this was a "purgatory" timeline. That bottom of the ocean shot set us up for something that wasn't quite delivered.
In reading some of the first few reviews of the finale, I came across this paragraph:
They made up an entirely new story for Season 6 as a pretext to abandon the main narrative that has dominated the entire series up until that point. By inventing "alternate reality" at the beginning of 6 and then focusing almost the entire final episode on it, Cuse and Lindelof escape the corner they painted themselves in over 5 seasons. To torture the metaphor, they basically said..."Well, this whole room is painted, but look next door! A room without any paint at all! What's going on over there?" And because they thought up a nice ending to THAT story, one that gave them an excuse to explore all the main love stories that have played out over 6 seasons, the audience is tempted to overlook the fact that essentially NONE of the main questions get answered, and none of the big plot points of the first 4 seasons are dealt with in any way.
I kind of agree that in the end, the climax served primarily as a conclusion to the newly invented sideways storyline of season six, but not to the rest of the series itself. I was expecting an ending that would kind of place the entire series in a new light--that at the end of the finale, I would be saying, "So
that's what the Island was all about." This ending doesn't explain or illuminate the importance or meaning or impact of the Island, or cause us to rewatch the entire series through a new lens--it just explains the significance of the "sideways timeline." (And not in an entirely clear way.) They wrote a great conclusion to the story we've been watching for the last four months instead of the one we've been watching for the past six years.
I understand the idea that the point of the entire series was just to bring these characters together so that they could forge a bond that would carry them through this life and into whatever lies beyond--that's a nice idea and hard to criticize (unless you are a supremely cynical person), but it does fail a bit as a resolution to the specific Island mysteries and questions of the past six years.