If only the movie had ambiguity. Veidt's inclinations are far too obvious throughout. I don't know if it's a bad performance, or bad direction, or both. But it's the opposite of what the film needed.
I liked Matthew Goode's performance better the second time I saw it but he's not helped by the fact that his character's "evilness" is not very subtley. Giving him a foreign accent was kind of a dead giveaway too although strangly it seems less pronounced on the DVD.
Malin Ackerman's performance is a little better too although she's still the weak link of the cast. Her character was badly hurt by the virtual elimination of her animosity towards Edward Blake. When it is revealed that Blake is Laurie's father in the movie, it has no dramatic punch at all because Laurie barely mentions Blake at all in the film, let alone hates him.
The 22nd Amendment is repealed in the world of Watchmen, allowing Nixon to keep running. Having won Vietnam in this world, he enjoys enourmous public support. Manhatten's presence changed everything in this timeline.
About the change at the end, with Manhatten being blamed for everything. I can see this forcing the worlds to work together but no way in hell would people be "holding hands and singing" with the U.S. if Manhatten had really done this action. They might work with the U.S. but they would likely be very bitter that the American government's ultimate weapon caused so much damage.