The cars are wrong.
Much of the story takes place in 1985 and if you look at the streets in the movie, you see a bunch of real mid-eighties models of cars. And that's wrong. Watchmen isn't just a superhero movie, it's an alternate reality movie. Two people in particular have made a huge difference in the world of Watchmen as it compares to our world: Dr. Manhattan and Adrian Veidt. Because of Dr. Manhattan, Nixon is still president in '85, Vietnam is the 51st state and everyone drives electric cars. And they're funky looking cars, not to be mistaken for what we had on our roads back in the day. Here's a little glimpse: