Now before I get started I want to try and bridge the gap a little between the haters and the apologists and to say that if you give it a change this final was everything we were promised, although you need to give it several watches through and use your head a little. When i first saw it I was disappointed, like WAS THAT IT? But having watched it 3 times now and having been re-watching series 5 on DVD I think that most people have over simplified the ending and so been left disappointed.
First a few pointers that this ending was thought out and not just thrown in at the end. If you notice in series 5 when Hurley is visited be Ana Lucia and Charlie, they are wearing almost identical clothes to those in the alti-verse, is this a hint to the fact that this alternative world is outside of time, and can run parallel to ours? Think about all the other times characters in Lost were visited by ghosts, Boone appearing to Locke for example.
Secondly, and this is the most important i think, the Church at the end, is the same one they visit in season 5 to meet Eloise and find the island. it is where the "Lamp Post" is and how she finds the island, in that church! So the church is a symbol of finding the island. Now i see the alti-verse as a kind of shared dream state, probably powered by the island, or by the effect the island had on them. They all had baggage they had to "let go" of and so could not remember who they are or each other (Jack wanting to be a good father, Sawyer wanting to do good, Locke married to Helen but still haunted by his father) and only when they found something they had loved or cared for in the real world did they remember. They must have also remembered that they had died and were called to the church. Why? Because the church symbolises finding faith, hope, love and THE ISLAND. At the end Christain opens the doors and there is a bright light, so everyone is like "oh it was purgatory and they go to heaven, how lame." But where have we seen a bright light before on the show? When Desmond switched the fail safe, when Ben moved the island (re-watch that episode in particular because the effect they use with the light is pretty much identical) and so the light at the end is the heart if the island. If the light in the island stands for the human soul or spirit (Jacobs mother says there is a little bit of the light in everyone, and if it goes out on the island it goes out everywhere) then the losties souls/spirits are being rejoined to the island, maybe to become part of the source (think the end of the matrix when the dead Neo is taken back to the source to restart it) maybe that is what makes the heart of the island the souls/spirits of the people who have been there?
Many were annoyed there were no answers given in the final, but most were given before. We now know how Jacob came to the island, how the smoke monster came into being and if we read between the lines can see the sense of the light. Jacobs brother said that many people had dug wholes on the island, maybe some time before people dug down and found the source, then plugged it so only a bit shone through and then they created a pool. Why? Well again Jacobs brother said he would use the wheel to mix water and light so he could leave, when his mother asked him how he knew this he just said he was special. She looked shocked and did not dismiss his idea, so obviously he was right, and the pool ( water) must be needed as well as the light. When Jack has put the stone back in, nothing happens until it starts to fill with water again, showing that Man in Black was right (maybe he talked to more ghosts or just knew, like Locke did for somethings or Walt. In Lost some people are just "special")
Anyway I think that makes for a much more poignant ending as we go full circle and they return to the island in some form, if you don't believe me re-watch seasons 5 and 6, and you will see it all falls into place.
Please feel free to comment now....