I didn't HATE it. Paul hated it.
1) actor playing Rorshach nailed the part. great casting.
2) it was a nice-looking move from a production design/art direction standpoint, especially the Mars scenes.
1) I would argue that in the book, "Watchmen" is Rorshach's story- in the book and the movie, he frames the beginning and the end of the story. The movie made it more like it was Laurie's (Silk Spectre) story, which I think was a mistake.
2) too much Nixon, Kissinger and other "look, it's an alternate history of the 80's!" characters.
3) A sex scene and a graphic violence scene (the one in the prison) that really didn't need to be there to move the plot along.
4) weird pacing and omissions of prior plot points (Adrian Veidt, New Frontiersman) that are important to get the ending.
Yes, they changed it for no apparent reason. The new explanation of the ending and what causes the results are unsatisfying. Even in this alternate world, I don't think the public would buy the explanation given.
If you haven't read the novel, you might be less harsh on the movie. If you have read the novel, you'll probably think it got some things right, but not enough to be a satisfying adaptation.
The Rorschach test done in the movie is administered wrong. (I can't remember how it was in the book). The tester is supposed to hand the cards to the subject, who is free to turn them on their sides or upside down before responding. If the subject turns the cards, he/she gets extra "points" for initiative (or perhaps, is penalized if he/she DOESN'T turn them).
Something else most people don't know about Rorschach blots- of the 10 cards, 5 of them have color(s) in addition to or instead of black ink. The blots are supposed to be secret, so the ones you see in movies are not the real ones. I have a set of the real ones because my dad was a clinical psychologist (you have to be a psychologist or psychiatrist to buy the blots) and I got his when he died. Here's what they look like in outline form (colors are described, and variations in blot shading aren't displayed) if you're curious: http://www.deltabravo.net/custody/rorschach.php
Also yesterday. birthday dinner was thankfully better than The Watchmen. Today is all about laundry and gym!