Much of my yesterday was spent discussing Lost with those who loved the finale, those who hated it, and those who just ended up feeling like they didn't get what had happened. Let me put myself firmly in the camp of those who loved it. Yes, there were still unanswered questions, but as Jimmy Kimmel so eloquently said, "If you care about the answers, you missed the point."
Let me do my best to explain what happened and why I am satisfied with the ending despite not getting my three questions answered.
We have to approach understanding Lost from the correct context. Thus, a few things are important to remember as we look at what was going on.
First, this show has always been, is, and will always be--at its core--about characters and their emotional baggage or "hang ups." The first few seasons showed us what those hang ups were and how characters reacted poorly to life's little hiccups because of that baggage. The middle to later seasons showed us growth on the part of the characters with some progressing further or faster than others. The final season showed us the characters finally letting go and moving on (at least most of the characters).
Second, the medium of flashes--Flashbacks, Flashforwards, and Flashsideways--is how we learn about the characters baggage and growth. This functions through both parallels in characters' past or future lives to what was going on in the island and through contrasting behaviors that were different from what we were seeing on the island.
Third, while Flashbacks were in the past and Flashforwards were in the future, Flashsideways existed outside of time; the characters were dead already and in a state of purgatory during the Flashsideways. Obviously this means that the Flashsideways were occurring after each character had died in reality, so it could be correct to call them Flashsuperforwards, but the events of the island still seemed to have an effect on characters during the Flashsideways (i.e. Jack's neck bleeding after fighting Locke) so somehow they were both far in the future and yet connected to the now.
Being about character growth and ultimately learning how to let go, The Island, whatever it was, served as the catalyst to understanding the characters. It prodded, pushed, poked, and provoked them and the characters reacted. The Island thus provided the scenario where the characters were forced to face up to their issues, confront them, and either move on or succumb to them.
So What Happened?
The final season of Lost took us to that place where all of the characters were ready to let go of their emotional baggage: purgatory. Characters continued to live in an "empty" existence--all were dissatisfied in one way or another--but they had no memory of their previous life (or "real" life as I shall call it. Purgatory will be called "side" life). Remembering real life was triggered by coming into contact with someone important from real life and sharing in a deja-vu-style moment with them. Once that remembering occurred, then it seems like most characters were ready to accept what was and cannot be changed and move forward with the person/people most important to them. The obvious exception was Ben who had begun a relationship with Alex and Rousseau but whose relationship with them was not yet fully developed to a point where they could all move on together. There is something important in that notion of moving on together that should not be overlooked in understanding what happened in Lost.
"John," you say, "that is all fine and good, but I still don't understand what happened. Desmond was pulling stone corks out of light holes and Jack and Locke were killing each other. What was all this Protector stuff and why did The Island need it?" These are great questions, but unfortunately I don't have time to get to them today. I will do my best to get to those tomorrow, but the truth is, I believe it is pointless to try and understand all of that stuff unless we first have a clear understanding of what drove the show and what was at its core: characters letting go and moving on. The Island, its powers, its protectors, etc. were NEVER as important as the Characters that learned to overcome the weight that held them back. So the first answer to the question of what happened is: Jack & Kate, Sawyer & Juliet, Locke, Desmond & Penny, Charlie & Claire, Hugo & Libby, etc. etc. learned how to let go of their issues and be at peace.
The point is: the characters found resolution. Their experience on the island helped them to do so.