OK. I'm not going to say that someone's wrong for not liking the finale, because it was based around a kind of crazy conceit which would not be to everyone's liking, and a lot of stuff was left unanswered or unexplained (but see below). And there are ways around some of the problems which may or may not be appealing to everyone. But here are some comments.
I guess you've probably gathered this, but the final season was basically split between two 'worlds' -- one where the main characters are on the island trying to achieve various conflicting things (wanting to get off of the island; wanting to defeat what had become the series's main bad guy, a smoke monster who had assumed the form of John Locke; wanting to find other characters on the island and rescue them; wanting to save the island and possibly the world from destruction) and another world where the island had sunk, the plain crash in season one seemed to have never happened, and the main characters' lives were different in significant ways.
In the season finale, the events on the island reach their climax (the smoke monster is killed, the island is almost destroyed, some of the characters die while others manage to escape the island and a few stay on to take care of it) and the events in the other world turn out to have happened far, far in the future in an afterlife where the characters had to work some things out before going on to their final (unspecified) destination. Over the course of the season some of the characters wake up to what's going on and try to bring other characters to a similar revelation, and at the end they gather in a church where they are led to wherever it is they're going. I think it's also said that they all go together because the most important parts of their lives were together on the island.
(Oh, except I should say that there are other legitimate interpretations of the finale -- a lot of people initially thought that it was implied that everything after the first minute or so of season one was a hallucination in the mind of a dying man, for instance, although I think mostly people have backed off of that idea.)
One thing that is not explicit in the series but which I think makes sense is that this 'alternative world' doesn't start at the precise time we first see it at the beginning of the season. For some characters it may have started much earlier, if that is what they needed. The time in that world at the beginning of the season is significant because it's when the characters start to move towards each other and towards recovering their memories of the original timeline.
Levy thinks that Locke (who almost committed suicide last season, only to be murdered instead by Ben Linus minutes later) should not have been happy to recover his memories. But this overlooks two things: (1) In his own little purgatory he's worked out many of the issues that were causing him problems in the original timeline, as Levy notes, and the whole purpose of that was to prepare him for this moment and (2) one of the things that he learned when he regained the knowledge of the other world is that he was fundamentally right in his beliefs and that his actions did in fact bring about the salvation of the island. (This was a particularly effective moment for me, because it hadn't yet been revealed what the other world was and I wasn't 100% sure that that world's Locke wasn't also the smoke monster, so seeing him recover his memories was particularly unexpected.)
Levy is also unhappy with how Jack relates to his (real) father and (fake) son. In most of the series Jack's father is portrayed as an alcoholic, a miserable human being and a terrible father, so their immediate reconciliation at the end of the show doesn't initially ring true. But there's some backstory here -- before Jack's father died (in real life), he had given up drinking and was trying to reconcile with Jack. Jack rejects his overtures and his father fled to Australia, started drinking again, and ultimately died.
In the other world, Jack and his son are similarly estranged at the beginning of the season, and at one point in the season Jack apologizes to his son, promises to do better, and the son accepts his apology and from there through the rest of the season they are much closer. This models Jack's final reconciliation with his father. Again, the purpose of the stuff that happened in the other world is to prepare the characters for the point at which they regain their knowledge and prepare to move on to whatever the next step is.
(Incidentally, one of the reviews Levy links to is unhappy that Ben gets to apologize to Locke for killing him -- in the real world you don't get do-overs, the reviewer says. And that's true but I think it's also significant that in the end Ben decides he's not ready to go on to the next stage and has more left to do in the world the others are leaving behind. He and the Widmores seem to be the only characters who know that they're in a constructed world but choose to stay there, although there are other 'real' characters who are also in the world but haven't yet received their personal revelations.)
(Also when Ben said he was staying behind and all the other characters went into the church I had a moment of thinking of how funny and wrong it would be if shortly after everyone went in the church it blew up spectacularly. Wouldn't put it past that guy!)
I think the island part of the final season did provide more answers than many folks give it credit for, although I agree that there's still a lot of stuff (including the whole Ben/Widmore backstory that Levy refers to) that's unexplained or underexplained. And it's frustrating that a lot of the final season was taken up introducing new backstory and mysteries rather than explaining things that people were wondering about. In retrospect you kind of have to wonder whether all the stuff at the temple -- a location full of people that was introduced and then destroyed -- was worth the episodes spent on it, for instance.
The 'other world' stuff wasn't about explaining anything, it was about providing character resolutions and making you cry.