SPOILER ALERT.
First off, I want to make clear that as I write this, I have had the unfair luxury of having read my colleagues' excellent analyses of the finale - both fantastic articles in their own ways.
THE EPISODE...
I'm going to focus more on the last ten minutes or so of the episode because I have little to say about everything that came before it - other than I found it thrilling. My jaw fell when Jack and Locke were fighting on that cliff in the rain - it hit that precise combination of epic scale and rich character-driven context that I loved about the show.
The last ten minutes are difficult to judge because it essentially tries to answer a number of huge questions at once: we found out what the flash-sideways was, and what was 'real' and what wasn't 'as real.' Most importantly, we found out the ultimate fates of our characters - in more ways than one.
Like I said, I had the advantage of reading my colleague's reviews. I think they're the perfect snapshots of the inevitable disagreement among fans. I think they're both absolutely right on some counts.
WHAT WORKED
In the positive, I agree with Dylan that Lost was very much a character-driven show. To memory, it was the first anthology series that disguised itself as a serialized show: most episodes were self-contained flashback/forward/sideways stories about a single character - chained together in an addictive fantasy serialization. To this extent, I'm among the fans that loved the sideways flashes that we've gotten this season. In serialized storytelling, there's only so many times you can tell a story about the same character without fundamentally changing who they are - so much so that the writers eventually had to go forward in time in order to give us new stories about these characters. The sideways flashes took all the elements we've come to know about the characters and presented them in a new way. Ben was an excellent example where the sideways flashes told us how to see the gentler and lovable side of one television's greatest villains.

In this sense, the last ten minutes of the series finale was fulfilling. We found out that the sideways flashes were a limbo/purgatory construct created by these 'lost' souls so that they could help each other remember what made life worth living. I found this to be a pretty novel idea. The sideways flashes were initially an annoying and implausible super-mega-happy ending that I can now tolerate because we now know they are unreal wish-fulfillment and fantasy. It was all happening because the characters needed to have a 'happy ending' in order to accept their deaths and move on to the afterlife.
Butler is right; it would have been unacceptable here if we had gotten a real happy ending. But I don't think we got that. Yes, the characters get a happy ending. But what we the audience got is the reality that there wasn't a happy ending. The final shot of the deathly-silent wreckage of the plane shows us what we really got: a story with a very real sense of death and tragedy. It turns out every character that we saw die really did die (except when Sayid died and came back, which still infuriates me). Likewise, we can presume that every character that survived (like Hurley and Ben) eventually died at some point because we saw them end up in that sideways-purgatory construct. It doesn't matter that they all died at different times, because as we heard, there is no sense of time in purgatory - a very fitting rule given all the time-travel the audience has been put through. What mattered was that they needed each other in order to individually move on to the afterlife.
WHAT DIDN'T WORK

But what kept this series finale from being fully satisfying was that it confirmed the skeptics' assumptions: the Lost writers never had a plan. Sure, I fully believe they always knew the series was going to end with Jack's eye closing as he dies. But it seems just about everything else in the series was tossed in without any regard as to how to resolve the increasingly growing mess of plot devices. By the time we approached the series finale, there was no doubt that they would be unable to answer all the questions we still had. This is why I absolutely hated "Across the Sea" - the episode with the crazy witch mother and young Jacob. Go back and watch that episode - it was only a couple weeks ago. From a storytelling perspective, it raises a million frustrating questions that would be impossible to answer in the series finale - so much so that our entire understanding of Lost would have been better served if we had not spent one of its final hours watching a crazy lady with magic powers kill some random Spanish woman and brainwash her two sons - all for no apparent reason. The fact that Lost couldn't ever answer its own questions truly weakens the series as a whole, and highlights the ultimate shortcoming of the series finale. Butler put it best with his Polar Bear example. In essence, we got HOW things happened (DARMA), but never never never WHY things happened. And that's really dissatisfying for a show that was about 'lost' individuals finding purpose and meaning.
BOTTOM LINE?
Like I said, I went into the finale knowing that they wouldn't answer most of the questions. What I wanted was closure. I would've been super-pissed if Lost had ended on a mysterious cliffhanger. Yeah, a cliffhanger worked for The Sopranos, but there was contextual and artistic support for that ending. The difference is that Lost made a promise - it was telling one big story, and that story needed an ending.

We got an ending. And it wasn't really the awful super-happy fanfiction-flavored ending we got at the end of Harry Potter where the good guys win and everybody loves each other and names their children after deceased comrades. It was an ending that embraced the tragic reality of death, while providing that sense of hope that beyond death lies some supernatural means for us to achieve the happy endings we'll never get in real life. Framed that way, it's actually a pretty fitting end considering Lost's themes. It's not a perfect ending, and might never be truly satisfying given the outrageous questions we still have. Moreover, hating this ending is not an invalid opinion - there are plenty of good reasons not to like it. But it was an ending with closure, which was what I mostly wanted. When Jack closes his eyes and brings the show to its end, I knew that narratively, Lost was finally over.
To be honest? That's probably enough for me.
-Aaron-