Yeah, so that happened. Thoughts, spoilers, etc below the fold.
In many ways, we couldn't have expected much better. The show's emotional hook was twofold: well-done characters grappling with issues of faith, trust, love, death and irrevocable choices in the face of uncertainty and their own tragic past; and the continuing unfolding of new mysteries. By the end of Season Two, this had essentially become untenable. They needed to start paying off the mysteries of the island, not just stacking weirdness upon weirdness. And so in terms of that second thread, in many ways it's been a long, slow downhill slide from there.
I could go over all the threads that were never really paid off--why the hell were they stealing the children? (Yes, yes--they had fertility problems. But they weren't adopting them, and they also weren't experimenting on them, so aside from the fact that the directors needed them out of the way in order to solve the continuity problem as they grew . . . what the hey?) Why wasn't Michael in the church? (or did I just miss him?) Where was Eko? Where did Jacob's mother come from? Why did "the place that they created" involve bizarrely fooling themselves?" WHAT THE HELL IS THE NATURE OF THIS ISLAND?
But that was baked in the cake for some time--maybe from the first episode. To some extent, the producers of Lost needed to choose between paying off the human drama, and paying off the sci-fi/fantasy/ghost story part; I think there's maybe a reason that those sorts of stories suffer on the character side. When what we might call the "technical" stuff is driving the plot, the characters necessarily become less rich, because complex characters who develop in unpredictable ways get in the way of making all those little jigsaw pieces fit together. Science fiction geeks are naturally mad, but I'm not sure that the producers made the wrong choice--especially when you think of what it took to get this series made, very expensively, on network television. If you love The Prisoner or Lord of the Rings or Star Wars, you're really not in a good position to complain that the background world doesn't all resolve into some neatly comprehensible set of rules.
Certainly, by Season Three, there wasn't much possibility of salvaging the technical side, because the writers had spent too much time wandering around in limbo, never knowing when the series was going to end. They couldn't develop any particularly complex backstory and unveil it in a dramatically coherent matter, because at any point they might be cancelled, and need to wrap things up quickly. By the time they had a multi-year committment, the thing was so muddled that they faced a choice between trying to salvage an almost nonsensical technical plotline, or developing characters who were, in fact, very compelling.
So though the final season does not work very well as a wrap-up of the mysteries--a fact which has been obvious to me since the first episode--I think Tyler Cowen is right that the final episode works very well at throwing us right up against the fundamental tragedy of life. One of the most appealing features of Lost is that there was no "main character immunity", and it's possible to view the last scene as a really rather blunt exposition of the tragedy of life without that happy assurance:
I view the show's cosmology as reflecting the existence of all possible universes and we get to see, and live with, a few of them. That includes the universe where they all die in the initial crash, the universe where they all die in the hydrogen bomb explosion, the universe where the hydrogen bomb creates an alternative reality, the universe where there really is a miraculously surviving "Oceanic Six," the universe where the main island narrative happens, the universe where it is all a dream of Jack's, and bits of others as well. This Leibnizian move "explains" the show's numerous unanswered questions, such as those about the lottery numbers and many more. It was possible, so it happened, toss in the anthropic principles as well.
The most striking moment of the final episode was when Locke tells Jack, quite sincerely, that he does not in fact have a son. The question remains how the different universes fit together or interact and in some manner it seems they do. The final episode is extremely effective in bringing out the dreamy and speculative tones of many of the previous episodes.
Most of all I viewed the ending as tragic. It was not mainly about any particular account of the metaphysics of the island. It was about how few couples had the chance to actually live together, love together, and stay together. The perfect reunions of the couples in the "we're all dead" scenario only drove this point home. I found this contrast moving.
Because Lost was good at character, we actually got attached to these people, and the last scene highlights how tragically brief their life was. Sure, they're all together, but most of them barely were in real life--and the contrast is all the more effective because in the end, we don't really understand what the purpose of all this was. If there was any at all.
And of course, that throws us also against the tragedy of our own lives. We are none of us immune, but we start life with the illusion of it--as Hazlitt said, "To be young is to be as one of the immortal gods". I'd say that we spend the rest of our lives fighting to maintain that illusion, in the face of a ruthless universe. It's little wonder that despite the show's many flaws, I spent the last fifteen minutes with my arms wrapped tightly around my fiance, and my tears flowing freely. But then, I'm a sentimentalist at heart.