As nice as it was to see all of our characters "moving on" together, I think I'm not the only person that is wondering why so many important people in our characters lives, lets take Helen as the easiest example, exist in the ALT but are nowhere to be found when our characters move on. Other very important people in our characters lives, say Jin's father, are nowhere to be seen. This casts a pall of sadness over our characters happy resolution. Sure it's nice to see your good island buddies, but wouldn't Sayid miss his mother for example? Or wouldn't an afterlife for Charlie without his brother be disappointing?
My resolution to this is that other than our church Losties and a few others (Daniel, Charlotte, perhaps Eloise and Widmore) NONE of the other people who appear in the ALT are the "real souls" of those people. They are constructs that help our Losties "move on", but they do not have the subjective experience of the Alt.
For example, let's say Aaron is eventually raised by Claire in the real world, he grows up, gets married, has kids, and dies in a freak renovation accident at the age or 83. I would say that Aaron DOES NOT EXPERIENCE the Alt, he would just go directly to "the light" and presumably reunite with his loved ones in that place, rather than turning back into a baby surrounded by strangers he will never know. Let's say the same for Helen, Carmen Reyes, Clementine and all the other people who are related to/important to our Losties but did not have any experience on the island. (Aaron being an exception, but this is because his most important life experiences would come later in his life)
According to my theory the people who experienced the ALT were only the people who were so scarred and traumatized by their experience on the island that they needed an extra level of afterlife. Only once they accepted and "let go" of the horrors that they experienced on the island were they able to "move on" to the "normal" after life where people like the REAL Ji-Yeon, Helen, Jack's mother etc. are waiting for them.
As a final example, let's take Sayid's brother. Upon his death he doesn't suddenly begin experiencing an afterlife where he is married to Sayid's unrequited love. He goes straight into "the light" and meets up with Sayid in the "normal" afterlife. Sayid's brother in the ALT did not have any subjective experiences there, he is not real in the ALT in same way that Sayid is.
The best thing about making theories now is that there is no new information coming to disprove a theory. After years of theorizing only to have our dreams crushed repeatedly, we can now simply decide how we want to view the events on the show, and no one can "prove" that the interpretations most satisfying to us as individuals are wrong. So if you like the above theory, nothing can stop you from watching the show with this in mind. If you hate the above theory, nobody can "prove" that it's true, and you can think of an interpretation most satisfying to you.
Namaste