Major Plot hole: Ozymandias reveals that his plan was to create World Peace by making the Soviet Union and the United States ally with one-another.
In the graphic novel, a teleported giant squid is used to make it look like aliens are attacking Earth, thus making all mankind unite forces against an outside threat. In the film however, Ozymandias uses nuclear explosions with Dr.Manhattan's energy signature to do the same job. But this cannot work. Dr.Manhattan is a United States invention. An invention that was used by Nixon in the Vietnam War to make them win and also to exercise their power over all other nations.
How can the Soviets unilaterally and instantly join the United States in an alliance when the tool of their destruction is an old American Weapon ? This is very unlikely, and it is even more surprising that the most intelligent man on Earth did not think about it.
Major Plot hole: Rorschach is killed by Dr.Manhattan because he knows he will reveal Ozymandias plan to the whole world. But since he can see in the future, why didn't he do something about Rorschach's journal that was in the hands of a newspaper employee ? Surely Rorschach's death is pointless if his journal gets published.