To be fair, part of the reason Lost started off as being so intriguing was that there was so much mystery involved. Polar bears, the black smoke, the foot statue and so on. An then as the series went on, they had to explain everything. I saw an interview with Dominic Monaghan (Charlie) where he said it was only meant to last about three seasons and the writers had an ending for it. But then ABC rolled in and said "we've got good ratings, write a few more seasons". So they had to keep it going, introducing new characters, the concept of a time travelling, disappearing island, a nuclear bomb, the oceanic six that got off the island but then came back and all that jazz.
Then season six; they had to wrap it up somehow. And I can't think of another way they'd do it. So considering they had to explain all that and conclude six years of mystery, they did an okay job of it. The whole parallel universe/alternate reality thing is a little far-fetched but so was the rest of the series. I won't give the ending away, but it does make sense and is quite clever if you look at it from the right angle.