It was a failure and a MASSIVE cop out. I'm sorry, but if you got wrapped up in all the emotional "pulling at the heart strings" moments in the finale, you did exactly what the writers hope you'd do... forget the fact that the show stunk and cry a bunch.
The emotional moments and character development was the point of the show and you seem to be missing it. The mystique of the island and the mysteries made it cool. All of the major mysteries have been answered for me personally. A lot of the other mysteries are akin to what "the force" is in Star Wars. I just don't understand how someone could watch 100+ hours of the show if you hated the characters.
To each his own, but it seems you watched the finale just to act all uppity about the writing. I know this is a strawman, but I'd be willing to bet the same people critical of the ending would be just as critical of the answers given.
Who put the cork in the island and knew to do that?
You telling me that's not important?
I love the characters and their development, but that is not what the majority of people tuned in for last night.
I don't think it's important to the story that was told, just as the adventures of Hurley and Ben after Jack's death weren't important. Sure, the show could've said "a group of Egyptian ex-slaves washed ashore and created it to prevent evil mystical energy from continuing to pollute the Earth", but then it becomes "what's this mystical energy? Where did it come from? How did the slaves know how to make the cork? Why is the cork so special?" etc, etc. Same deal with the statue, or the fertility problems, or where Jacob's "mom" came from, or what the smoke monster actually was, or why a bird said "Hurley!" for no apparent reason a few years back. You'd need 2 or 3 extra seasons of pure exposition to wrap everything up, and you'd wind up with something very bloated and unsatisfying.
I saw the show as being about a group of characters plucked out of a crossroads in their lives, placed in an unfathomable situation, and how they reacted to it as they peeled the onion apart layer by layer. Just because the onion was never fully dissected, and some of the character arcs ended in unsatisfying ways doesn't mean the story wasn't an enjoyable ride, which is the whole point of an entertainment show.
Was it the best-written show in TV history? Nope, though its best moments were some of the best I've seen on television. The most coherent? Nowhere near it. But it was fun to watch, was incredibly ambitious, and had some of the best characters and plot hooks I've ever seen on a TV show. That's pretty darn good in my book.