I liked it, overall. Here's some comments
1. MUSIC - I HATED how they didn't just stick with a completely original soundtrack. I hated how they stuck other/popular songs in there. It felt like they'd throw in a song simply because it had one word in it that matched the current theme, went along with the quotations in the graphic novel, or took place in the 80's. I think the worst thing they did was throw in "Ride of the Valkyries" (spelling?) while Dr. Manhattan is blowing up Vietnamese soldiers. It turned a serious scene into something I LAUGHED at. Whether that was intended or not, it should NOT have been like that. I have no problem with the music itself, but how they used the music...as in, I really wish it was just all original (like LotR, for example). It really can make or break the movie...or at least make things feel weird.
2. Fighting - Everyone seemed to have super strength...it needed to be toned down.
3. Acting - Some of the acting was great, some was just borderline decent.
4. Blunt - The movie is blunt. All subtleties are gone...
5. Ending - I really preferred the movie's ending to the graphic novel's. I honestly hated the graphic novel's ending.
6. Change - They changed some things that really didn't make it any cooler...or make sense. Archie now comes out through the river...what? That added nothing, and it makes more sense for it to come out of a local abandoned warehouse (I believe that's what it was).
7. Sex scene - Yes, it was originally there, along with accidentally hitting the flamethrower. But this was just too awkward.
8. Intro - Music ties into this...the intro was cool, but won't really help clear anything up for anyone who hasn't read the GN.
9. Backstory - Lacking...Also, it's hard to pick up on the story if you haven't read it (general consensus).
10. Penis - I don't see why everyone is complaining about it. It's never the center of attention. That's how it always was in the GN.
I can't think of anything else in particular. I know those are all mostly negative comments, but assume I enjoyed everything else.
I don't see why people dislike it so much, outside of not having read the GN. I can see why that'd cause problems. I personally would have enjoyed it much more if they had just stuck with an original soundtrack.