LOST: "The End"
It's the ending of Titanic all over again; wherein Rose dies and goes back to the time in her life when she was happiest, on Titanic with all the people she knew and loved on that doomed cruise, with Leo and in her youthful, beautiful Kate Winslet body -- she returned to her version of everlasting bliss -- the time when she was happiest and most fulfilled in her life.
It's the same thing here, just a little different.
All the stuff on the island happened. The flashbacks, the flash forwards, the return to the island. It all happened.
The story began with Jack and it ended with Jack. As Christian told him, he had to let go. Ironic, given how close we all thought Jack was to doing precisely that in the preceding episodes. The most important thing these characters did happened on the island. The finale was not about the deaths of each of the characters. It was about the death of Jack.
That's why all these people he was friends with, all these people he was close with and loved were there. THere is no "Now," as Christian said -- they all died eventually. But tonight, in that church, it was all of them as Jack needed them to be so he could let go and move on.
Ironic that Ben, in death, recognized that he probably shouldn't have gone in.
Further ironic that each of the other characters, upon seeing their happy futures in the sideways universe/heaven, recognized that they'd see Jack soon enough.
It did have meaning. Jack helped save the world. Hurley became the next Jacob. Ben finally got a job because someone believed in him. Jack finally saved the day, and the last thing he saw was that plane soaring away before he died, right where he began this journey.
No man is an island. Jack needed each of his friends' help to get there. For Jack, his happiness was with seeing everyone coupled off and happily together. No Charles Widmore, no others or mercenaries. No Michael or Walt, because Michael killed Ana-Lucia and Libby in cold blood, at least in Jack's eyes. This wasn't the end-all be all of heaven; it was just the people he lived with on that island that mattered most to him.
Given all the repetitive imagery throughout the episode obviously meant to evoke previous scenes we'd scene, I was not at all surprised to see Jack approach the empty coffin and was almost expecting to see him open it to find his own dead body in there -- a literal copy of The Prisoner's famous final revelation ... but what we got was even more profound -- literally, spiritually and figuratively.
Kate, Sawyer, Miles, Richard, Frank (Frank!!!!) may or may not have made it back to the mainland. It doesn't matter because the ultimate irony of the show is that it's about JACK, the character who wasn't even supposed to make it past the pilot. And whether or not that Ajira 316 flight crashed and everyone aboard died or if they all lived to reach their ripe old age, the point is once they died, they came back to help Jack let go of life and move on to the afterlife.
As for the flash-sideways and what they meant:
I think we can now establish that when Juliet detonated Jughead at the end of "The Incident", two things happened.
1) The bomb detonated, and combined with another time flash, the released energy sank the island in 1977. This theory is supported by the fact that in the flash-sideways universe, the island, complete with Dharma barracks, Tawaret statue, and Dharma shark are all under the sea as Oceanic 815 flies by (as seen in "LA X."
2) The time flash also, coincidentally transports Juliet, Jack, Sawyer, Sayid, Kate, Hurley, Jin, Rose, Bernard, and Vincent back to 2007.
The flash-sideways are the result of the nuke going off and sinking the island, killing Jacob and the Man in Black in 1977, and thus neither of them being able to affect any of the lives of the Oceanic passengers.
Now, whether or not this means that once the island sank the entire world ended is open to debate. It could just be that the world continued on, or it could be that the world did end and that everything from the flash-sideways is the afterlife. Or, it could be that the lives of the Oceanic passengers were better for the lack of interference from Jacob and his quarrel with the Man in Black.
Either way, the dawning realization of the characters in the flash-sideways segments all season has been united in one singular purpose -- to bring Jack to his end in the prime universe and to allow him to let go of life and move on to a happier existence.