Never read the GN, but it was a pretty good movie. The action was neat, the storyline was really cool. Some parts dragged (Rorschach's "joke" near the cemetery... yawn...) but mostly it flowed nicely. Apparently they changed some of the ending, but it was still good (and, from what I heard of the ending in the GN, more believable).
Rorschach's joke is one of my favourite sections in the novel. The film didn't convey it very well. I was disappointed. The new ending is an improvement, though.
What was left unsolved, IMO, was how the heroes in the montage near the beginning died. When Rorschacht is first going on his investigation and thinking someone was picking off "masks" I kept thinking they would be tied back in to the story, but they never were. I guess instead it was just to show the tragic life cycle of heroes.
Silhouette (the lesbian) is shown murdered with her girlfriend, Mothman (with the moth wings) is shown being dragged into an asylum, Dollar Bill (with the dollar bill on his chest) is shown shot to death with his cape stuck in a revolving door. It skips over Captain Metropolis and Hooded Justice, but to be honest I think that was because of those character's very obvious homosexual subtext. They get dealt with mostly in literary interludes in the GN and don't play a significant part other than filler.
So, not really unsolved. It was the screenwriter's way of incorporating all the background which you otherwise find in-between chapters of the novel into a simple and informative opening montage. What's unsolved is what happens to Hollis Mason. That was one of the saddest scenes in the GN, and it's just cut completely out of the film. I really hope it's in the extended edition.