This most likely will not make sense unless you’ve read the book an seen the movie.
1) In the book, Ozymandias was so driven by his Utopian vision, and his megalomaniac vision of himself as bringer of this utopia, that he slaughtered half of New York to make it a reality. In the movie, Ozymandias is so driven by megalomania, and the Utopian vision that justifies it, that he slaughters half of New York. It’s a subtle difference, but I much prefer the book on this count. It makes Ozymandias more human, and thus more frightening.
2) In the book, Dan “Nightowl” Dreiberg and Laurie “Silk Spectere” Juspeczyk, seem like lost souls who find meaning in the costumes they wear, and when they have sex in the Owlship, it’s one of the few points of human connection in the books dark vision. In the movie, Dan and Laurie seem like overgrown children who find escape from themselves in the costumes they wear, and when they have sex in the Owlship, it seems like a pathetic juvenile masturbation fantasy come to life. It’s one of the darkest moments in the film. I guess that the film in this respect is far more successful deconstruction of the Superhero myth, but a meaner work.
3) The people and their dramas around Madison Square Garden provide a nessesary counter wight to the rest of the story. If Watchmen is about the futility and danger inherent in our myth of the hero, it needs the power and nobility of everyday life, the quiet nobility of the psychiatrist, the cab driver strugling with herself, the power of these stories in the face of their extermination, to avoid decending into bitterness and cynicism. The movie, which is without them, is much much weaker for it.