A lot of complaints about some of the costumes' lack of faithfulness seem to me to be unfounded, or at least making a big deal about something that's not such a big deal. Here's why I think so.
1. The costumes in the novel were NOT IMPORTANT, except to bring up the very minor point that "Superhero costumes look silly and it takes a certain type of person to wear one". The costumes still look silly and I'm sure the line about them looking ******ed will be in the movie. And that's all we need.
2. Alan Moore, I'm sure, didn't think more than five minutes about whether Nite-Owl should be gray-and-brown or brown-and-brown, or whether his cowl was attached to his cape. The only thing that's important is that dash of ridiculousness, and both Gibbons and Snyder's costume designer have provided that. I'm sure someone on this board is going to come out with a treatise on why the shape of Dan's goggles is a deep metaphor for his inner psychological turmoil and his emergence as a full-fledged human being, and why having square goggles destroys the character. But let me take a pre-emptive strike and say: No, you're stupid.
3. The costumes in the novel are pretty much on par with what a typical comic book costume was considered to be at the time (well, more golden-agey for the Minutemen). These costumes in the movie look like what we moviegoers today conceive of as typical costumes. The point is that the audience shouldn't disbelieve the costumes until the story makes that point explicitly. It should be "You know, superheroes do look kinda goofy when you think about it", not a right-off-the-bat "Are you kidding me?".
4. The costumes, on the whole, are about 80% faithful by my reckoning. It's really not reasonable to demand more, especially when everything but the costumes looks spot-on (to me, and most people on this board).
5. Gibbons approves. Yeah, we're all entitled to our own interpretations of his work, but his opinion does matter, and it matters more than any of ours.
I'll grant to the complainers that Ozymandias looks a little too villainous in that picture, however.