This show could have been brilliant, if it hadn't spent so much time trying to convince us it was brilliant. The problem with Lost is that it does not come through on it's central premise--- to explain the unusual events on the island.
First of all, the show is basically a revved up version of the show Mysterious Island, based of the Jules Verne book of the same title, which I believe was a Canadian television show. But Lost did bring a lot to the table that was promising, if not wholly new. The acting was generally good, and every now and then there were outright fantastic performances. The drawn out emotional montages in the first few seasons actually worked quite well. And the mysterious elements of the Island were intriguing: Smoke monsters, 'Others' weird scientific anomalies, possible conspiracies, etc... but then, once it becomes apparent (around S4) that the writers can't explain anything they've set up within the established reality they've created... they start adding alternate realities. They even jump the shark and add Time travel. And this wouldn't even be problematic for many viewers --- these are good sci-fi staples, and Lost always had a sci-fi appeal --- but rather than explain anything with these clichés, the plot is simply muddled. By the last episode, it becomes apparent that neither the 'alternate realities' or the insertion of time travel into the story actually explain anything, but rather complicate an already complicated story into obscurification.
The final episodes of Lost, where 'all is to be revealed' leave plenty to be revealed. Even if you're willing to accept that there are god-like beings (or maybe gods, it's never clear) or simply 'magical' folk running things, this is not enough; you must accept that these gods act without any clear purpose, insisting on the legitimacy of 'rules' that they can't explain. And once you accept that, then you see that those 'rules' don't actually have any purpose, either. Anything an audience holds to in effort to make sense of the reason the island matters, or the reason these supernatural god-figures who can't leave the island (but do) should matter will be disregarded within a few episodes. In the end, Lost, which was so promising, doesn't really mean anything.
And worse, as it loses any attempt at making sense of itself, it revels in drawn out sentimentality. All the cut scenes become interchangeable dumb expressions of people refusing to explain themselves, or acting shocked when they're promised that they're about to be told something.
Why can't women on the island have babies (even though they do)? This is never explained. What 'sickness' on the island drives people crazy? Never explained. What is 'the light'? Never explained. What reality of the many glimpsed in this show is 'real'? Never explained.
There's sooooo much unexplained; and what does actually get explained is usually just explained poorly, and upon some examination falls apart. But all this aside--- is it a good story? Is the action worthwhile? Yes and no. The action scenes are good eye candy most of the time, although the actions of the main characters become more and more contrived. People begin initiating fights for no reason, and the characters seem to be acting very willfully against their own interest, as often pointed out by more reasonable characters; but it doesn't matter; these type-A characters are driven to act by fate itself, apparently. In fact, many of the 'main characters' (Jack, Kate, John, etc.) become irritating and unlikeable, despite their continual hero status. But action? Yes, as implausible as it all is, some of it is good action. There are good plot twists, but these are entangled with repetitive clichés; guards are always easily overcome; something will always give one second before the explosion; someone always got away from that explosion, even though you didn't see it. It eventually just gets boring.
In the end, is Lost is the best-made dumb show ever. It severely underestimates the intelligence of it's audience, and this is a mistake when you're selling what's supposed to be a 'smart' show.