I'm not saying seeing how the characters move on after the story is absolutely necessary. I just did not feel that the events in the sideways reality this episode provided any sort extra insight or closure to the characters, for the most part. It was feel good, but hollow and empty in the end.
I fully expected that the sideways reality would become the real one after Smokey was killed and the island was saved, with everyone inheriting memories from their time on the island. If this actually happened and the characters were given screen time to thoughtfully consider both versions of their lives we might have a much more real sense of closure. Also Sawyer could be with Juilet, Jack could be with Kate (assuming she works out her legal problems), Jin and Sun would be alive and able to take care of their kid, Sayid could be with Shannon, Charlie with Clair etc. They would be able to take with them the best aspects of both worlds, true redemption for their trials on the island. Jacob would come off as much less of a dick for condemning so many to death because he could not make up his mind. And there would be a sense finality in knowing the island, being sunk at the bottom of the ocean, is probably protected for good.
But the one question they actually bothered to answer this episode destroyed one of the only things that did not really need an explanation, the point and purpose of the sideways reality. It was the "afterlife" all along. Great. So all the feel good reunions we got to see this episode were pointless, especially those involving dead people. Am I supposed to feel happy that Jin and Sun got to see their newborn son? They are still dead.
They could have really pulled some of this together but they sabotaged themselves for no good reason.
As for the island plot, yea it happened but did so in as cliched and by the numbers method possible. No explanations onto the nature of the island were given, a question that most certainly demanded some kind of answer, or even insight. All we learned was that the light comes out off a hole in a cave that needs a cork in it else it goes boom. This all powerful evil force that could destroy the world if let out can apparently be contained by a 100 pound rock.
I guess it was a magic rock, that explains it, just like everything else on the island.