First off, having watched the movie with someone who had never read the comic, I can confirm that according to her 'It was kind of hard to follow what was going on sometimes.' Which is a sentiment I've seen some question.
And now, my thoughts/takeaways
I liked most of the casting except for Silk Spectre and Ozymandius. Silk Spectre just wasn't written the way she was in the comics, take of it what you will. She worked alright in the context of the film I suppose. But Ozymandius was poor casting all around. He looked too young and not world weary enough. In the comics, almost everytime you're given a close up of Veidt he's wearing this immensely knowing, sad smile. His character was always the perfect encapsulation of hope and despair, and a lot of that was lost more by the actor than the writing.
Some of the changes I fealt worked fine. Having Doc Manhattan admit his awareness of the Tachyons and potential nuclear war earlier in the story worked alright. But some I wasn't so keen on, like having Dan warn Adrian instead of Rorschach. That served only to inflate Nite Owl II's importance which was kind of contrary to his personality. Dan is supposed to be kind of an impotent nobody in the beginning of the story. Plus, having Rorschach not meet up with Ozymandius denied Rorschach of some of his good insults, and having him ignore one of the original costumed adventurers in his warning run doesn't make sense in the context of his character. Cutting out even the slightest acknowledgement of Hollis Masons death cripples Dan's character and motivation, and wouldn't have taken that long to add to the movie. I considered it an odd cut because they seemed to be interested in inflating Dan's importance and it is exactly that sort of thing that would do it. I'd also like to note that, having watched it with someone who hadn't seen the movie on the way out she was asking 'I couldn't tell, where they supposed to have super powers? Because I didn't think they were but in some of the fight scenes they seemed to be flying around a bit.' Which I will concur with. I understand that it makes the film more visually interesting, but I always envisioned Rorschach as more a stealthy, shadowy kind of guy and not a flip out style ninja.
One of the cuts I was upset about personally, but didn't matter much in the grand scheme of things, was Rorschach leaping out of the refrigarator. The story moved fine missing that scene with Moloch, but just having that whole buildup and have him freaking leap out of the fridge was awesome in the comics.
Of course, it must be said, Squid versus Manhattan. I completely understand the change to Doc Manhattan but I have a problem with it. The reason why it worked was because it allowed them to cut out a lot of the side story they didn't want to film, which is fine for an adaptation. The Squid only works once you realize all the stuff that's been building up to it (the off hand news stories about artists dissapearing, Ozy's work with Bubastis, etc.), and to keep things to a more principal cast shifting the weapon to something associated with them is fine. However, I don't exactly see the Russians as being alright with the U.S. after Manhattan blew up a bunch of major cities. You mean to tell me the Russians weren't thinking 'You guys created this monster, you held him over our heads for years, lording your superiority over us, and then you lose control of him and he starts destroying the planet?' The alien threat worked because it wasn't associated with any country, but the Manhattan threat still has a lot of ties to the U.S. I don't think the Russians are just about to up and forgive. Also, Veidts tracks would be a lot harder to cover, it's not as though his work on free energy was secret, they show him talking about it with heads of business and the press. So we're to assume that they day he finally finishes his free energy generator (which he's been hyping up) the world blows up and no one puts two and two together? At least with the Squid he went through the trouble of picking a plan so ridiculous that it would be very, VERY hard to just come up with it off the top of your head. The Manhattan plan isn't bad, but it lacks some of the insane brilliance of the Squid.
There's a lot of stuff I expected to miss, but some I was suprised. I don't mind truncating the story because I saw it coming, but little things that wouldn't have added to the run time kind of made me frown. For example, right after the Comedian accuses Doc of losing touch with humanity in 'Nam, as Doc stands by observing the dead woman while Comedian walks off, Docs leg is phased through an overturned table for no reason other than it seems to be in the way of him standing where he wants. I always saw that as a great little point of his 'He's so out of touch with people he's acting less and less like them.' I was also suprised to miss having Veidt performing gymnastics for charity while Dan and Sally awkwardly fumble on the couch. It was always a great scene that really fleshed out Ozy's character while simultaneously really belittling Dan's (which may have been why they cut it, with their obvious Dan inflation interests.)
With the art direction, I was a little upset to see things go too sleek. I just thought the costumes could've been a little more 'dirty/homemade' looking for a lot of the characters.
I liked the movie, but unfortunately, the comic is a celebration of the medium itself, and the film was just never going to be able to capture all of that. As adaptations go, it's probably about as good as you could get.
Oh, and for anyone who haven't seen it. Saturday Morning Watchmen