I love the Watchmen, it had me awestruck from the first page. I am not very experienced with graphic novels and I don't want to overstep my own knowledge on the matter, but to me Watchmen defined the entire genre. It works so beautifully (and in my opinion would have been impossible to put to screen) because every frame speaks to the genre of the comic book so brilliantly. Now a lot of the times when someone says something speaks to something else I want to punch him in the face. But I didn't know how to phrase it any other way and so I will try to explain myself.
At the beginning you see a character carrying a sign reading The End is Nigh or something like that. The character keeps showing up over and over and to be honest on first reading I didn't put it together that that character is Rorschach. And later on Ozymandias tells the story of Alexander cutting the knot, if you go back a few frames you'll see the characters cross in front of a giant painting of that story. The power of Watchmen as literature is that you can jump through the frames and find connections across chapters and pages. You get the sense that the story doesn't move linearly so much as it creates an entire breathing universe that you can walk through as you wish.
Not having read it in awhile, what I remember mostly is the book's investigation into morality. Moore carries us through this world and leaves it up to us to pick a character and take their side. He presents the issue of justice as an intangible and meaningless word, a concept that is completely at the will of the individual. And he invites us to be the individual. How can we say Ozymandias is truly an evil character when his motives for good are given to us as genuine? But then again, didn't Hitler think he was making the world a better place? Part of the greatness of the book was Moore's ability to present so many tragic heroes with different identities and different motives inside the chapters of one book. And they were all so memorable (except maybe that girl who never really jumped off the page). He explores so many different themes and ideas without overcrowding the book. And the science fiction part was amazing while not being overbearing.
How you know that this book is truly great is the amount of conversations it inspires. There is so much in there open for so much interpretation.
As to your discussion of the endings Arthur, I actually preferred the movie's ending (it was the only thing I liked about the movie in fact). I thought the movie's ending was simpler and made more sense. But as to avoid repetition I already lamented about the Movie in this
post and also discussed the ending.