Mark me down for "disappointed", as well, though I fully expected to be (so I guess that's sort of a contradiction). I give Cuse and Lindleoff the benefit of the doubt and believe that they knew the major plot elements from the beginning. And if all you look at are the major plot elements, they did a good job. Season 1 was magnificent all around (character development, mystery, action, humor, etc.) and it set us up to care about these characters and want to solve the mysteries of the island. To have the final season take place half in the real world and half in purgatory (or whatever you want to call it) and nudge the characters we care about to ultimately accept their flawed and incomplete lives for what they were is good stuff. Even the fact that many of the island's/show's mysteries weren't answered goes to strengthen this ultimate resolution... that life doesn't end when you've solved all its mysteries, it ends when it ends... that it's not about the destination, it's about the journey... etc. Not the most ingenious, creative or original story to tell, but told in a unique way.
However (and this is a four-toed statue sized "however"), the creators caved to network pressure along the way and turned what should have been a 3 season show (max) into 6 seasons worth of advertising revenue, and it became way too filler heavy. There wasn't enough depth to the characters to write material to span 121 episodes. Hell, The Sopranos, which is the best episodic, character driven drama of all time and came from the minds of vastly superior writers, only spanned 86 episodes. Let's be honest, you take away the island, the time-travel, the other gimmicks, you place these characters in the really real world... you've got a pretty weak character narrative and a show that no one would have been interested in. Cuse and Lindleoff are talking out of their asses when they say "it's always been about the characters". Bullshit... it's always been about the characters AND the freaky shit they're going through and trying to figure out.
So they ended up developing banal plot lines (did you care about any of the individual stories in season's 3-5?) and relying on more and more mysteries... and it's one thing to accept that 20-30 mysteries go unanswered to strengthen the final resolution, it's another thing entirely to accept hundreds of mysteries (many brought up in the final season) go unanswered. I would imagine that a re-watching of the series would feel extremely unrewarding, as most of what is presented doesn't matter (by the creator's own admission) or is uninteresting.
As for the actual final episode, it delivered closure for the characters, but did so in a "chick flick" sort of way. I imagined housewives across the nation blubbering every time someone "remembered". They also (finally) managed to seamlessly move from real-time to sideways-time, which went a long way to maintaining a decent pace to the episode and created some cool moments. The action was pretty lame... never felt the same excitement from season 1 during the Jack/MIB fight, the cave sequences or any of the "these are supposed to be a big, exciting deal" moments. And speaking of the cave of light... meh, looked more like a theme park attraction than "the spigot from which all that is evil must be prevented from spewing forth".
One last thing... the unanswered questions. Like I wrote 10,000 words ago, I can appreciate not answering all of them, especially considering the "lesson" the characters (and the audience) are supposed to walk away with. But some stuff just needed to be answered ("Why was fertility/Walt/Aaron so important early on?" comes to mind... or "Who was making the food drops?"). Without answers to those more basic questions, the mystery angle of the show is left too wide open to be satisfying.