There's only so much you can say about this book without using the word OWWWW.
The last couple of chapters will hit you where it hurts the most, right at Vin and Elend. This book is just one revelation after another. Most of the things that were brought up in previous books get resolved in a satisfying way. A few things remain a mystery, as they should.
Although everything works out in the end and the world is reborn, the journey toward the end is a painful one, even moreso than the previous two books. During several moments all hope seemed lost, and for a brief moment, it seemed perhaps things would've turned out better if Rashek had lived. Those were dark times indeed. You know you've hit rock bottom when you start thinking of Rashek as a good person who only wanted to hold the world together. (I still don't buy it though. If he was such a decent guy, he wouldn't have ignore the racial, class, gender, etc. disparities fracturing society during his rule. If he was such a decent ruler, he would have strived harder for unity and equality, instead of dividing the people and holding them under with his ridiculous laws. He had a thousand years to work shit out, and he chose complacency. He deserved what he got. But that's neither here nor there.)
Back to this book. The world of mists and ashes had to end before it could be reborn into a world of blue skies and green grass—the one that Kelsier and Mare dreamed about. Themes and symbolism in this book are greatly influenced by Judeo-Christian beliefs. If you're not familiar with or interested in religious symbolism, you'd be able to gloss over the repeated mentions. Though not obvious, they do get annoying when you pick up on them and start counting the number of times they show up. Sazed's loss of faith was expected to make his reclaim of it more dramatic when the time came for him to realize that he is the Hero of Ages. I had a feeling it couldn't have been Vin, that would've been too obvious.
What I didn't expect were the deaths of Preservation and Ruin. Could gods, or forces of nature or whatever they were, cease to exist just like that? Fall into a blackhole and vanish? I also didn't expect Vin and Elend not live to see the end—way to hit below the belt there, Sanderson. But they're not dead, neither is Kelsier. Or Rashek? And where is Sazed? What will become of this new world? (Time to pick up Alloy of Law)
Also, that Ruin... what a sneaky bastard.
Somebody would need to watch over the world, care for it, now that its gods were gone. It wasn't until that moment that Sazed understood the term Hero of Ages. Not a Hero that came once in the ages.
But a Hero who would span the ages. A Hero who would preserve mankind throughout all its lives and times. Neither Preservation nor Ruin, but both.