The end, for me, left a lot of questions but I think life in general leaves a lot of questions. The central theme to that being "live together or die alone". Ergo, we are all on this rock they call Planet Earth together...and why do we do the things we do to one another? Is there a reason underneath it all? Can we ever understand irrationality when it's played out across the stage of humanity?
As far as the Sideways universe and the Island Universe: I think there has to be a basic acceptance that parallel reality (note: NOT parallel universe) IS possible. That there are black holes and worm holes and string theory and the theory of singularity. That we can exist in this world and other worlds at the same time. That time itself is NOT linear (Farraday had pointed this out) but shaped more like the Donkey wheel with spokes pointing IN (as opposed to OUT) to a singularity of time which occurs for all humanity.
To bring the alternate universes together requires a singularity - an epic event that occurs to the center of each universe, that is, to each of us as we are ALL the center of our own universe and that singular, epic event is death. The show theorizes that death is that singularity that unites all the disparate realities we experience. I can't remember who pointed this out last night but one of the Losties said that a single event when viewed from different angles can have very different interpretations. This is key to the theory of the parallel universe where events are close but not quite the same...the view is different depending on where in time you are looking from.
Desmond says he isn't afraid of where he will go next as he is lowered into the light. He knows that he will be reunited forever with everyone he has loved. This was key to understanding the end.
The end, I felt, focused on Jack's singularity although I do believe that as Christian told him, everything that had happened had been real, everything that he thought he had done, he had done. I think we saw everyone's reality throughout the show not just Jack's. But in order to fully tell the theory of the singularity and how that unites the realities, the writers focused on Jack.
As Christian explained to Jack, everyone dies. Some before us and some after us. This was critical since I don't believe everyone in the church was dead. For example. Kate, Lapidus, Claire, and Miles were not dead in my opinion. But in Jack's reality, as Desmond had said, he was reunited with all those people whom he had journeyed with on his trip to his singularity.
I think Cuse and Lindeloff looked at near death experiences where people say they go to a white light and often meet a cherished love one who guides them in. In Jack's case, this was Christian Shephard. Each person had their "constant" which I think now is the person who leads them to the singularity. And we also now know that just because you're MY constant doesn't mean I am YOUR constant. Last fall I had started to see that there was something to these "destined" relationships...that there were relationships between characters that defied time and space, like Jin and Sun but I couldn't get past the romantic aspect to really flesh that out. Now I see that your constant is the one who will guide you in to the singularity where you will be with those most important to you or as Christian said, you don't ever have to do this alone.
There was a joining of all religious traditions as evidenced in the stained glass windows, another singularity when all are joined as one. The Losties themselves arrive at the church in Jack's reality and are joined in his singularity. I am sure that as each reaches their own singularity the same type of scenario plays out for them. For some that scenario played out many, many years earlier and for others, it won't happen for years to come.
But Jack's singularity is REAL because as I said, time isn't linear. Time weaves in and out and undulates and sometimes occurs parallel to itself over and over. It is only with each person's moment of singularity or "Island Awareness" does that time explode into a single point where everything you have ever known is finally joined together. According the Theory of General Relativity, the Big Bang that created the universe occured at a moment of singularity. This, I think, aligns with what the show portrayed at the end. Christian said we create these places for ourselves - they are our singularity point or Big Bang. It doesn't mean an end but rather a beginning of something as yet undefined. You could think everyone was dead...or just starting a new world (or worlds) in which to live. Desmond kind of implies this theory which brings the whole idea of reincarnation into play as well.
I think overall it was an outstanding way to wrap up the series. I don't think, in hindsight, the questions I thought needed to be answered were ever really relevant. Sure, it would have been fun to know some of those answers...but in the bigger picture it wasn't really what the show was about. It was about the human condition, how we grapple with our imperfections, how we reconcile what we have done and what we think we should have done, how we live our lives despite the terrible things we can do...and how we can change. Overall, it was definitely about the overwhelming theme of potential and how as humans all we can do is try to do our best for as long as we can. Once we have accomplished that, we can move on to what's next.
As Desmond says, "see ya in the next life brothah!"