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To me, the ending was incredibly original. I was very relieved that it wasn't a cop-out like "They were dead all along" or "it was all just a dream".
I don't know how other people interpreted it, but here's how I (and much of its fanbase) saw it. In the LOST mythos, when one dies, they are sent to a sort of purgatory where they have to prepare to move on to the final afterlife. This purgatory mirrors their actual life to some extent, except for the fact that they eventually must meet up with all of the people that were important in their first life in order to make the final realization of who they were and what was, and is, important to them. Once these requirements have been filled, and all of them are ready, they can pass beyond into the last afterlife (or maybe even another stage - who knows?). This is what the flash-sideways was for the Oceanic survivors. It occurs on an entirely different timeline from theirs, so there is no before or after - hence, why they were all present despite dying at different times, and why flash-sideways works so well as a name for the thing.
As for what went down on the island, that's a bit more complicated and more fun. The MIB/Smokey was, until now, trapped on the island, because he had been damned in a sort of strange way by his brother after being thrown into the Source and corrupted into something he was not previously. Gone Mad From The Revelation, if you will. Basically what he's been trying to do this whole time is kill off the candidates without having direct contact with them - as part of his unstated pact with Jacob, he cannot harm any of the actual chosen people directly - and use them as stepping stones to get off the island and wreak havoc in the world that he desired for contact with when he was "alive". To do this, he actually manages to get the light out in the middle of the island, cutting off its power and basically dooming the world. But what he didn't think about before going through with this was the fact that doing this would cut off his power, too, and make him just as mortal as the rest of them rather than simply a physical manifestation of an almost entirely nonphysical being. Jack managed to kill him and afterwards turn the island back on, therefore saving everyone from certain death. Because enough people have died already, seriously, what the hell kind of good guy is Jacob? Couldn't he have given the plane a slightly smoother landing? So Hurley becomes the new Jacob and eventually dies with the rest of them, and Jack dies very poetically.
I don't know what I hoped to accomplish by summarizing that, I kind-of just felt like doing so. I'm sure everyone will go on hating the finale as usual.
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