A hundred year war has torn the world apart. One of the four races are all but completely decimated. And the attitudes of nearly 1/3 of the world for over a hundred years are being expected to change abruptly.
Admittedly, many citizens of the Fire Nation were victims of its oppressive culture. Or at least a fair portion. Yet, still, there are many others who thought the war was nothing but beneficial. Those who spread propoganda throughout the empire throughout all the war. And certainly those who believed it. I think we're all sure they wouldn't accept the values being instilled upon them by a man who has been branded a traitor. It also doesn't help that this man is barely even that, at 16 years old.
So will there be revolts? No question about it. We don't know how long the coronation of Zuko was after Sozin's Comet, but we can assume it was around 3 months. What kept them? Was it organizing everyone to come? That could be it, but there would be protesters because of it. Aang's insistence on peace would assuredly force them to come up with a peaceful solution, but what if there was none. What if the gaang decided to ignore it? But these people aren't afraid to lose their lives. They probably have lost family in the war, and have been raised to support it since they were children. Now it's all over? On a day when the Fire Nation was supposed to bask in its greatest glory yet?
Then there's the Earth Kingdom. Essentially the same, but on a different side of the spectrum, as they were the ones being terrorized by the Fire Nation, the Earth Kingdom and Water Tribes don't want anything to do with the Fire Nation. Most anyway, as we've seen from Zuko Alone. These people certainly won't be against peace, as they have only really been on the defensive (from what we've seen anyway) throughout the war, but they won't be attempting to blend cultures anytime soon. Too much bad blood, unfortunately.
Can we really expect the war to just be forgiven? That some people were wrong and because they're sorry now (as few will be -- this generation didn't cause the war) they should just love each other? We don't know what propoganda has caused some people to hate the surrounding nations (although Zuko does say in DOBS that they were raised to believe the war was altruism, the Fire Nation simply spreading their culture throughout the world) but feelings won't be eased.
You could say Aang and Zuko's friendship could 'set an example' for the rest of the world, but will the Fire Nation that easily respect a leader who, not long ago, was the object of hate for the entire country? The Earth Kingdom and Water Tribes will likely respect Aang and be peaceful, but still hold hate towards the Fire Nation. And as long as there is hate, we can't honestly expect reconstruction to start.