What are some major differences between how the Allies treated Germany after World War II and World War I?
The major difference between the way Germany was treated after these two wars was that Germany was essentially punished and then left alone after WWI but was remade under supervision after WWII.
After WWI, the Allies did not occupy Germany or split it up or anything like that. They allowed Germany to continue to rule itself. What they did do, however, was to punish Germany harshly. The provisions of the Treaty of Versailles were meant to harm Germany, not to help it.
After WWII, the Allies did not allow Germany to remain independent. Instead, the Allies completely took control of Germany and ran it themselves. However, at least in West Germany, this occupation was rather benign. The Western Allies wanted to rebuild West Germany so that it could be economically strong and politically stable. This was very different from how they treated Germany after the First World War, when they punished it harshly and then left it to its own devices.
Thus, Germany was left to run itself after WWI, but allowing it to retain its independence was not as beneficial to Germany as the Western occupation after WWII.
Consider the terms of the Treaty of Versailles, contrasted to the terms of the Nazi surrender ending WWII in Europe.
The Treaty of Versailles blamed Germany for virtually all of the war, Nazi surrender only put the blame where it belonged. The Treaty of Versailles required Germany to pay the rest of Europe for the damage that was done, the Nazi Surrender treated all the parties in WWII as victims, proportional to damage suffered (i.e. Germany and France were absolute victims as both countries were virtually destroyed, the US wasn't a victim as it was virtually unscathed).