Alan Moore refused involvement, either way. Demanded his name not be used in any promotional materials. Refused royalties, too.
Too many horrible or inappropriate adaptations of his work, it seems. Plus it could be argued that as a deconstruction of the comic book form, "Watchmen" is fundamentally lessened by adaptation to another medium, the same way that a deconstruction of the novel in the form of "Breakfast of Champions" failed as a movie and a rap song about that painting "Ce n'est pas une pipe" would fail no matter how good the rapper involved in the project.
It's not to say that there couldn't be merit or some translation of the work but... the parts that are direct commentary on the form are kind of irrevocably lost.
I mean, though I'm fighting with a lot of ideas right now, I have one that only works as a film because it's a commentary on the form. The general narrative structure COULD be a short story or a comic book, I suppose, but some of the meta-commentary would be lost as some of the metacommentary is buried, in this example, in the form of intentional continuity errors.
So, I dunno. There's a lot of stuff in Watchmen where the shapes of the frames or the page layout are part of the message. Mirrored sequences, the use of Rorsharch's journal as thought bubbles, the pirate comic within a comic... I don't know. These things don't MEAN the same things in a 24fps live action presentation.
I mean, Moore is awesome-crazy to begin with, he's just opposed to the idea idealogically but... especially "Watchmen".