First of all, I think the ending was both a success and a failure, depending on what you wanted from the show, and what you thought it was.
As science fiction, it was a failure. We never find out the mechanics of the smoke monster, the light, the electromagnetic "properties" of the island, etc... Why Tunisia? What caused the time travel? What was going on with Faraday's rocket? If one was looking for rational explanations of these phenomenon, one would not find them. Personally, I came to realize that the show was not science fiction at least a season ago, maybe even earlier than that. I'm okay with that... I don't know why the One Ring makes you invisible, and I don't care. It's fantasy, and fantasy can be done well.
As character drama, one could argue that it was a success. Jack comes to a resolution with his need to make things right ("fix" them) and he has his denouement with his father. Sawyer finds his way to justice and redemption, and reunites with Juliette (having already resolved his need for family by joining the Dharma Initiative and playing house with her). Sun and Jin get to have the baby together, Kate gets to save a mother (not her own, but Claire), and so forth. Hurley gets to help, Ben gets to serve, all that. Fans who were caught up in these aspects of the story are well satisfied, and so, I believe, are the writers who have long said that they wanted to solve the mysteries that the characters cared about more than the ones the fans may care about. Fair enough. But there is a third aspect, which is where the ending falls short for me.
It is the big-picture narrative, the critical motivations that drive the plot overall.
- For example: What does Charles Widmore want? What would be a "win" for him? Why did he send Martin Keamy and the mercenaries? To kill Ben? To capture Ben? To kill everyone but Ben? Does he want to destroy the island, save the island, control the island, or... what? The Mother says that men want more of the light, but is that what Widmore is, simply the greed of men? Why does he bring Desmond back to the island; Desmond being the only one who can pull the magic stopper and... destroy the island? Save the Island? Why does he fake the crashed underwater plane with all the dead bodies? I don't know, and I wanted to know.
- For example: What was the Dharma Initiative doing, and why? I know they were doing "research", but into what, and in what way? Why have people sit and punch numbers into a console every 108 minutes? Was it "faith", that is, they wanted to see if people would do it? That would explain the notebooks that go nowhere. But no, it turns out that something really bad does happen when you don't push the button, so is it "science"? If so, why have people type the numbers in like that? Seems pretty dangerous. You have a computer system, why not just automate the whole thing? Why the fake quarantine? What's the thing about the babies? Why these particular stations? Why do the "Dharma drops" keep coming after the employees are all killed and "the initiative was deemed a failure?" Who is sending them? Why?
- For example: What is Eloise Hawking trying to do? She knows that she killed her own son accidentally in the past, which must be her greatest tragedy... so she makes sure he becomes a physicist, makes sure the oceanic six go back to the island... to make sure it happens? Why? What's her goal? She seems exceedingly determined, but I'm not clear what she's determined to do.
The ending is lacking, for me, because it fails to address the key motivations and actions that constitute most of what actually happens in the narrative. We never really know why the man in black wants off the island. We're told that "everyone will die" if he succeeds, but why is that? Is it even true? I have no idea.
I think the writers realized at some point that they had written themselves into an impossible situation. They determined, I think in season 5 if not earlier, that they needed to get the audience to "let go" and "move on" when the show was over, and that this final season was essentially a 12-step program to get us weaned off the show. Unfortunately, this ends up in an emotionally-manipulative ending that is satisfying only to those who wanted that kind of thing; the lovers reunited, all that.
For me, it was disappointing. I still really enjoyed the show, and I'm glad I watched it. But I'm not sure I'm strongly motivated to watch it again, now that I know that so many of my questions will be essentially abandoned as unimportant, and that the climax of the drama will be totally about love, children, and the inevitability of death. Interesting topics, but not the show I thought I was watching.
So, a little bummed here.