Just Wow, I am blown away, this was perfect, truly nothing short of a masterpiece.
I am gonna try to explain what this "afterlife" is, cause I see a lot of people being confused as to why Jack has a son, or why Aaron is still a baby, or why Keamy would be there.
This afterlife first of all, does not exist in a specific time or space. It is a world that acts as if they had already dead. I take it as they sub consciously created this during the detonation at the end of Season 5. They all sort of willed this afterlife into existence so they could always remember each other as they were during the Island days. This is why characters who were alive were still in the church, such as Sawyer. Aaron was a baby cause it is how they collectively all remember him.
As far as David goes, or Keamy, these things were just obstacles, allusions if you will, to get them to realize everything. It was their test of "letting go". Also, I see people saying why did Charlie remember with Desmond but he had another awakening moment. We saw many times that there was a difference in the way they could remember. When Jack touched the coffin he pulled away and had a quick flash, but when he held his hand there he remembered it all. So that is easily explained.
So just think of it all has a place where time does not matter and it acts as if they had all died.
With all that being sad, I think it was great in that it ended with what the show should be about, the characters. I am kind of happy so many things are still up for debate, but I am extremely happy We have closure for these people we have loved for 6 years.