I never would have guessed that the island was actually The Black Lodge. The surprise cameo from Kyle MacLachlan ? Blew my mind.
I'd say the finale was 95% awesome, 5% so terrible that I'm unhappy with the rest of it.
I liked the story on the main island, I thought that Jack being the temporary guardian of the island with Hugo's more permanent acceptance of the role worked well. I liked the alternate universe (prior to the church), with everyone connecting to their other lives and re-connecting with each other. I liked the eventual redemption of Ben, even though I think he should have had to work a bit harder for it.
Basically, my problem with the finale is that they didn't resolve something I was unhappy about in a previous episode. I thought that the deaths of the Kwons was badly written. It felt artificial, like the writers were trying to make it into a romantic tragedy instead of feeling like the natural behavior of the characters. The whole Romeo & Juliet, die together instead of living apart thing just doesn't work for me given they had a child. As I understood her, the character of Sun wouldn't have wanted Jin to die and leave their child an orphan.
I was hoping this would be undone in some way during the finale, with the alternate reality providing a solution. Instead the alternate reality is some sort of afterlife, meaning that their child grew up without parents because of Jin's decision to die with Sun.
It bothers me even more given how significant parent issues were in the show. Many of the characters were significantly defined by their relationships with their parents or children.
If you take out Jin's death, or remove their child from the story, I would have been much happier with the finale.
My other gripes are fairly minor and probably rather common. (Sayid remembers his life after seeing Shannon, really? Nadia who? No real explanation of why The Others murdered the entire Dharma Initiative? Why did Kate becoming a mother remove her from the candidate list when Kwon remained on the list? Etc.)