The violence, for one thing. A large part of the problem is how the violence is used. The owlship sex scene for example occurs after Nite Owl and Silk Spectre don their costumes and come out of retirement. Prior to this, they try to have sex but Dreiberg is "impotent" and can't get hard while still just a civilian. This is all in line with the comic but Moore was making a commentary on the fetishism of sexualized heroes solving problems with violence. Snyder highlights the violence in exaggerated fashion and has it culminate in a giant orgasm in a very over-glorified manner. This doesn't mean that the movie couldn't be violent or that the sex scene needed to be cut but the way it's shot, prolonged, and the choice of music is offensively bad considering the source material. Snyder is saying "Holy shit! This is awesome! Oh my gawwwd you guys, everyone wants to be a swole superhero and fuck people up and fuck Silk Spectre with their massive dick!!!!!"
The change in the ending is also unfortunate but a lot more understandable. It makes more sense for audiences who don't want to really think about symbolism which is par for the course for popcorn movies. Watchmen isn't a "popcorn" comic book though so it's personally disappointing. The tentacle monster is a physical manifestation of the "other" that humanity is so prone to seek out and destroy. Changing that to Dr. Manhattan is convenient but misses out on some the subtext in the comic by making "the enemy" yet another man.
There's a ton of other stuff you could analyze but the movie is a very close transposition from the comic to the screen so a lot of the things Snyder misses is represented more in the execution and not so much the plot.
guek said it better and in more detail, but just compare any of the fight scenes in the comic to their counterparts in the film. The former are realistic, brutal, and often make the viewer almost wince looking at them; the latter are full of speed ramping, slow-mo, improbable feats of agility, and other stylistic touches that feel calculated to make the viewer go "holy shit, Rorschach is a badass!"
Not coincidentally, these are among the few scenes where Snyder actually seems to be having fun and not trying to slavishly imitate the source material, which makes it all the more striking how thoroughly they undermine the themes of the work he's attempting to revere.
While I'm not going to call Watchmen a perfect adaptation (Although I will say it's as successful as any other attempt I can imagine; fuck your HBO miniseries), I don't really agree with this point. I think it's more of a scapegoat that people have invented to easily explain away their actual problems with the movie, which I will get to in a moment.
The fight scenes in the movie are big and over-the-top and most likely there to keep the casual audience from being bored, but I don't think any action sequence feels superfluous to the actual story. They are longer and more drawn out than anything in the comic, but comics typically don't have long action scenes. Each panel is depicting a second in time, and so to do an action scene in a comic you're only going to draw a couple of the key seconds within the sequence, because nobody wants a flipbook in the middle of their comic. When adapting a comic's action scene into film, you're going to have to fill in the gaps.
Here is a fight scene from the comic:
And
here it is in the film, in which someone has cut out all of the intercuts with Dr. Manhattan's talkshow appearance. From the first swing, to the thugs hitting the floor, it's just about 40 seconds, most of which is in slow-motion (which is necessary to fit in with the Manhattan scene), so the whole exchange is really about 20 seconds or so of real time combat. All of which is bloody and disturbing. Much of the violence in the movie is heightened with lots of blood and loud sound effects of tissue and bones being crunched into, probably to emulate the vivid blood splatters of the original artwork and to recreate the perception of fast, harsh movement in reading a comic.
I haven't read the comic in years, but I seem to recall the prison fight being pretty much entirely made up for the movie. I can see how fans could take issue with that, but I don't feel like it does anything detrimental other than not be in the source material. It doesn't conflict with anything set-up in the movie, and logically it flows with the story. Dan and Sally are back in hero-mode reliving their glory days and so they beat up some rioting prisoners. It fits right in with their rescuing the tenants from the fire. Obviously, someone who read the comic will know that the scene was added to beef up the action, but it does so without conflicting with any of the subject matter.
You say that the action has an almost orgasmic quality that the comic is lacking, but I disagree. First of all it's an Alan Moore comic, it's not off-base to try to find some connotation of sexuality in literally everything he writes. But in Watchmen it's explicitly made clear that Dan is literally impotent until he starts getting back into his Nite Owl persona. There is this thrill and pleasure that they get from being superheroes. It makes absolute sense for a movie adaptation to accentuate this quality, and that can really only come out in the action or a character just saying "Man, I love being a superhero."
And let's not pretend that the original comic didn't try to make its limited action-violence aesthetically pleasing...
Two of the most beautiful pages in comics said:
These action scenes should reveal the euphoria that the characters are feeling. The enjoyment of the action is the only time we can truly relate to that experience. And
when the violence is less "heroic" then it should be played more straight.
Earlier I said that I feel like this criticism is something concocted for ease of critique. What I actually feel is the problem is more superficial. Following the release of 300, Zack Snyder suddenly had a "style," and that "style" was slow-motion, as well as those speed-ramping cuts that are kind of uniquely "his." I think this is what people ACTUALLY don't like about the action in Watchmen. While the rest of the movie is directly adapted from Alan Moore and Dave Gibbons' comic, the action is totally Zack Snyder. It pulls people out of the experience, not because the effect itself, but the unfortunate subconscious connection that people have that states: "Zack Snyder directed this." This thought occurs every time the action goes from slow to fast, and people hate it. That's the real problem with Watchmen's action.
P.S.: I actually saw the second post first and then went back to the original post without fully reading it. Upon going back to see if I'd adequately addressed all of the points, I realized that I didn't address the literal sex scene. Instead of going through the trouble of finding a logical place to insert it in my original argument, I'm just going to lazily shove it in here.
Complaints with the sex scene I've never felt strongly on one way or the other. Moore ended the scene with the flamethrower=orgasm sight gag. It was always going to be kind of awkward. I don't know why people have such a problem with the song, but then again, I actually like Leonard Cohen and was familiar with the original version of the song before I saw the movie. When I saw it in theaters there were laughs, but I figured there were supposed to be. While you can relate to a fighter's pleasure through visually stimulating action, eroticism isn't as simple, especially when it involves a man dressed as an owl banging a woman in a hovercraft. These two people are really fucked up. We're seeing two crazy middle-aged people who can't function unless they're running around in latex beating up criminals in 1985 New York. No movie featuring such a scene would connect to audience's on an erotic level, so the scene had to be filmed with an intent of absolute dissonance. It's the audiences chance to realize how ridiculous superheroes are as a concept.
Again. I have some problems with the movie. These just don't happen to be among them.