Finally got to see it last night, I was out of town for work.
I think most people have summed up the details of "what" happened.
The LAX timeline was "purgatory" of "limbo". They intentionally made it non-denominational with the stained glass window behind Jack at the church where he and his father spoke. You could see a Islamic crescent, a christian cross, a buddhist mandala, etc.
I think the use of the "light" at the church is meant to invoke the idea of the souls of the dead "going into the light". That motif has been used in countless other movies and is a common item reported in Near Death Experiences. So clearly they were moving on from purgatory/limbo to what ever the next stage was
Of course, this raises the question does this also say anything about the light on the island?
I think the show clearly established that the primary focus of all the characters was the conflict between good and evil. It was externalized in Jacob vs. MIB but each character faced their own inner demons and had to make choices in this regard.
It also clearly established that the light was somehow a focal point for this struggle. I'm willing to allow the writers to claim that the 'light", "power", "specialness" or what ever other term one wants to toss out about the island is their
MacGuffin. Actually, as I read the Wiki entry to explain the term, I see that this answer has already been added as an example.
I don't have an issue with this, every work of fiction is entitled to one MacGuffin, and since the essence of this story is how the characters respond to each other in their attempt to deal with the MacGuffin, it doesn't really matter
what it is.
It's your opinion, of course. I would argue that there's never been a show that even comes close to rivaling LOST's creativity and complexity, nor do I think such a show will occur for a very long time.
I would argue that Babylon 5 is one you could put in the same category, with one caveat.
One has to understand that the original plan was to have B5 run for 5 seasons. It was an independent production and not affiliated with any network.
Late in the 3rd and during the filming of the 4th season it didn't look like they would be picked up by any distributor for season 5, so Straczynski compressed season 4 and 5 into one to address the main plot points he wanted to.
It was only during the middle of Season 4 that TNT agreed to pick up season 5 so he had to change gears. As such season 5 was a little disjointed and didn't flow as well as the original 4 seasons.
So if one looks at the first four seasons, and a limited number of season 5 episodes, then I still think that is one of the best pieces of cinematic fiction ever done. That would include TV shows, movies, etc.