FWIW, Jay Leno uses "Lost" as source material for many of his jokes on The Tonight Show. In his view, no one has ever understood the premise of this series.

lets make it simpler!!
I simply loved ending, specially mirroring the last season with first. End at closing of the eyes of jack with vincent beside.
Island was a real thing with some metaphysics qualities. life on island was a real life. in last season, the flash sideways was about after the death of all persons. some of them died on the island, locke was killed by ben, some left island but eventually died at some point of their lives (as mentioned by christian that everyone dies at sometime). So after death, they didn't remember their lives but started imaginary life (you can say) as they wanted to be in real life (being successful business man, being a successful doctor) or they were at stage of purgatory , but none of that was real because you can notice that no one talked other alive persons, but always to each other. so when they touched each other, they remembered their lives and realized that they are dead and should move on to heaven.
Now who was smoke monster? obviously he was brother of jacob, converted into monster when jacob thrown him into that light cave due to some metaphysics phenomenon and wanted to leave the island by destroying it.
Only one thing remained unanswered that why it was important to save the island?? :)

By the time I type it all out, you could of watched the entire series by then.

Yea so there was this plane crash, and then Jack closed his eyes