So, you’ve got this Island. The Island keeps “hell” locked away from the Earth. The same power that locks away hell also imbues the Island with all sorts of magical characteristics. Since time began, someone has had to protect this cork so that hell would not spill forth into the Earth.
The protectors of the Island, once acknowledged as official protectors, could basically bend “the rules” of the Island to their whim. For example: Jacob loved playing games, and always wanted to create his own game, so he turned the candidate selection into a massive game. This process included the lighthouse from which he could pick candidates, the time distortion around the Island to keep other people from getting in, and most other vaguely magical things. It would also be what would create the Smoke monster from his brother: he needed an opponent, and once he killed his brother he lost that, so the Island created another force for him.
The protectors existed because humanity would inevitably discover the incredible force within the Island, and would always try to use it. Thus, civilizations had previously created objects that could tap into this source: from the teleportation wheel to the Dharma Initiatives experiments. This obsession would always be their undoing, as Jacob observed. Perhaps this was why he went out into the world to find candidates, instead of letting people continue to arrive.
Desmond was immune to this mysterious power at the source of the Island, which was vaguely related to electromagnetism. He was the only one who could go down into the source and remove the plug, which shattered the “rules” and began to bring “hell” upon Earth. So fake Locke could now escape the Island, but he could also be killed. The process nearly killed Desmond and it also effectively ended Jack’s life.
Meanwhile, the alternate universe was some sort of non-denominational purgatory, in which all of the important survivors had to let go of their past with the Island, and accept that they were all eventually “found.” Michael wasn’t there because he was still atoning, Eko wasn’t there for the same reason. Walt was not there because he had “let go” of his time on the Island when he spoke with Locke however many episodes ago that was. Ana Lucia and Keamy were still not absolved either, and Eloise Widmore understood that she was in a Purgatory, but did not want to leave her son, Daniel, yet.
I am slightly disappointed that the resolution is basically “this show was never about the mystery, it was about the characters, and they’re all happy so bye,” because I was never really compelled by the characters. Sure, half of the show was flashbacks and flashforwards and flashsideways completely separate from the Island and meant to be a character study; but for me, the characters were a little two-dimensional, and the dialogue was frequently hackneyed.
However, (and this is a big however,) Lost did something a TV show has never done before: it created a mystery that we were all a part of, and we all did come back to it, and the show came to a conclusion that made some sense in the grand scheme of things. I do not regret my time spent with the show, and I do think it has been an important experience and cultural touchstone.