Ok I've been wanting to make a post on Lost for a while. Considering I just did a rewatch it's the perfect time to do it. When you understand the context of the show, the ending(as well as the whole show) is perfect.

The main themes of Lost are karma and destiny. At the end of Lost, all of the characters are in a church together. Many fans took this as they all died and went to purgatory. And while in a sense this is true, perhaps our lives are purgatory as well. Perhaps in some alternate universe we go to the island with a select group of people we are all destined to meet. These are the types of questions that Lost wants you to ask.

The single most important event in understanding Lost happens in the season 5 finale. Juliet bangs a rock against jughead. Does it explode... perhaps it doesn't explode and the hatch is built over it(and jughead possibly becomes the fail-safe). Or perhaps even more oddly, they both happen creating a fork in which two different realities emerge.

The 2nd most important issue to understanding Lost is the hatch. 4 + 8 + 15 + 16 + 23 + 42 = 108. 108 in eastern mythology represents enlightenment... understanding ones destiny. As Jacob(and his mother explained) the light is the source of all things. A little bit of the light is in all things. And the hatch was the location of a breach in this electromagnetic force. The electromagnetic force started tearing apart reality if the energy wasn't dispersed. So basically the hatch sorted out how energy was released into the world. And in a metaphorical sense, it was the gears of destiny.

When Locke decided not to push the button any longer, reality started overloading. So to stop this Desmond turned the fail-safe key(and in an odd way, took over for the machine). Desmond became able to see the future, able to cross over between the different universes. Desmond became resistant to the electromagnetic spectrum(he existed outside of the casual events that held reality together). The survivors were running across each other frequently in the alternate universe, yet they didn't realize that they were destined for each other... until of course Desmond showed them.