Okay. Here we go...
If I were Zach Snyder, I would hope that everyone viewing my brand spanking new film would understand "Hey, I'm adapting a massive work of sequential art that was designed to be read (yes there's a difference) not passively watched. The original source material had not only comic-style storytelling, but also prose, another comic, newspaper clippings and a ton of other details that are simply not possible to be "filmed." Maybe that's why Alan Moore (the writer) states the work is absolutely 'unfilmable.'"
That being said, this film does just about everything a 2-3 hour dose of WATCHMEN can do. It tells the story. Characters are introduced, justice is dealt, squid is omitted. Based on the fact that no amount of comparison between the "graphic novel" (not accurate, it was a comic series (but that's an insult in Hollywood).) and the film will ever be realistic or relevant I've decided to be totally unrealistic and irrelevant. Here's what I perceive as the core differences, failings, and successes of the film as compared to the comic:
1. Amped up violence prevents Rorschach from being the truly horrifying creature the graphic novel creates. Yes, he's still scary. The problem is that in the comic ole Rorry was an extremist that flew only by his own rabid-dog tenacity and inability to let anything "go." In films, emotional content is a commodity. In this film that commodity is spent (in the form of violence) with such wreckless abandon that it makes the big "R" look as if he's just the most extreme, not the most insanely extreme. That creates a slight weakening of his purpose to the story. "R" as I'll call him (cuz we're tight, and he wouldn't mind) is heroism distilled down to it's essence and mixed with ultimate unwillingness to negotiate with ANYONE. He's an archetype, and an important one. Elevating the other "heroes" to even a similar level of violence erases a part of what makes his perspective different and previously unique.
2. The squid. While this isn't a real problem for me, the absence and replacement of the squid does change things. For me, the use of Manhattan as the bowtie shifts a story focused primarily on the ethical condition of man onto a vein about religion and it's purpose in humanity. I am however a much deeper person than you, so I'm sure you won't notice the shift. :)
3. Night Owl. He should have been chubby. Night Owl's weight is an important visualization of his mental state and overall normalcy. Patrick Wilson was great. He shoulda had a gut though.
Overall, I enjoyed the film a great deal. It does not, however match or even emulate the experience of the comic. For my effort, the comic was a new experience that completely caused me to rethink storytelling. With few exceptions (Zot, Batman:Year One) I would place it among the most influential books in my short and unnoticed time drawing comics. Lastly, Dave Gibbons eye for design and layout have yet to be matched. If you don't believe me, just look at ALL the marketing for the film just before you read the comics.
So, in short I had a blast watching the movie. As an experience though, the WATCHMEN series stands on it's own unfilmable or not.