That's not true at all. I skipped work this morning to watch it with friends, and I've never read the book (though of course I've known about it for years). So, here's my take (in rough cuts):
It was long, but for the most part it didn't drag. There were definitely some plodding bits, but I didn't check my watch until the 2 hour mark.
The action was generally really good, though I thought the bones-breaking-through-skin aspect was a little overdone. I didn't enjoy the gore.
The storyline was totally coherent to me - Maybe knowing the full backstory makes it seem like they give certain things short shrift, but from my perspective it held together well.
Holy crap, Rorschach is a badass. "You're locked in here with me"? Best line of the movie. Also, umm, there sure was a lot of dong.
As far as negatives go: The makeup for some of the characters was absolutely awful. Man. The biggest thing to me, though, is if Dr. Manhattan can manipulate matter at the atomic level, why doesn't he just take the cancer out of his ex-girlfriend and be done with it? Instead he flees to mars and lets her die. Weird choice.
The ending with the egyptian temple motif and all that felt pretty hokey - I don't know how else to put it. Ozymandias was supposed to be some sort of superb ultra-nemesis, but he really just seemed sort of flouncy and lame (except for being fast, apparently). The tiger-thing at the end was also really dissonant. We hadn't been shown a world with that sort of engineering, so it was strange to see it with no explanation at all.
I guess that's really the only other major negative; Nothing about their tech or their abilities is explained, except for Dr. M. Wtf kind of mask is Rorschach wearing? Does it respond to his emotions or something? It looks like cloth, but the blood comes out... wtf? How are they all such amazing fighting machines? Owl guy is out of it for a couple years, getting flabby and out of practice, but in an alley he beats the snot out of an entire gang. How?
All in all, I really enjoyed it. I thought it was a well-done film, for the most part, except for the bad makeup here and there. I know they had to compress a long storyline, which is where I assume explanations for abilities and equipment went, but if there were critical plot elements cut I didn't notice the gaps.