I'm sure a topic like this is a spit in the face to most of you here... so I'll gladly take it.
The movie did a few things different, and honestly they were for the better.
* Nigh Owl's suit - much more practical and realistic. Given the high-tech Owl Ship, it would make little sense for Dan to not spend the same amount of time on his costume.
* Adrian's plan - the angle with Dr. Manhattan is much more realistic and logical than building a giant squid. People already knew Dr. Manhattan's abilities. They feared them in a way. Even in a straight up comparison, he is far more dangerous of an enemy compared to a mere alien squid.
* Comedian - I think the movie version is just done better. You see a side that you can't get in a comic book. Jeffrey Dean Morgan adds certain small personality traits that make the character feel more complete. Plus making him older is a good idea. A 22-year old trying to rape a 20-year old is more believable than a 16-year old teen.
Well, I think that's enough to start a fire...
Let's have you all rise up and toot that a movie can never surpass the book in any way...
I hate to feed the troll, but meh.
* Clearly meant to reflect the tech-savvy side of Batman that Nite Owl is based on. However, the idea in the comic is showing that while Dan Dreiberg is a mechanical genius, he is socially awkward (and thus impotent unless wearing his cheesy-looking costume, which is still just as cheesy in the movie). Not to mention, most of us like/accept his costume change.
* The cancer frame was only a small part in Veidt's plan. Sure, it seemed somewhat logical to frame him for attacking the world in anger, but ultimately fails in comparison to the comic's ending. Why would Dr. Manhattan attack the Earth just because they believed he is radioactive and gave several people cancer? The idea of some being from another planet randomly attacking New York City is a lot scarier than something you already know. Come to think of it, did you even read the same Watchmen we all did?
* There was plenty of the Comedian from the comic that didn't make it into the film that made him a lot better. Sure, maybe visually he was an improvement, but in terms of character development the comic beats out the film. I don't know where you get that age span thing from, but what are you talking about? Rape is rape, no matter what age. Do you know what a pedophile is?
Also, that last statement, about a movie never surpassing a book, you're quite wrong about that. I can only think of Fight Club and 300 for example right now.