What we're two of the most important steps that macarthur took in japan following the war

1. Although it was a US/Allied government decision and not MacArthur's, the Emperor was preserved as head of state, was not prosecuted for war crimes, and although he eventually had to renounce his divinity (ie, the idea that he actually WAS god) the religious position of the emperor was preserved. 

2. He severely limited and controlled how war crimes investigations took place and how the trials were conducted. They were carried out before military tribunals, unlike the Nuremburg trials in Germany which were carried out by the most prominent legal people from the Allied states, in a specially set-up process. In this way, MacArthur and the US could scapegoat a limited number of Japanese for all the war crimes, while the Emperor was excused, and those who carried out barbarous germ warfare and chemical warfare experiments on (mostly Chinese) prisoners were also allowed free, in exchange for giving all their information to the US. The ongoing hunt for war criminals was given up very early, whereas Nazis and supposed Nazis are still being hunted down to this day. 

MacArthur's main contribution was to encourage US troops to behave themselves, forging a new friendship between Japan and the West. But overall not much of this was MacArthur's work and he mostly swanned around self-importantly, which is what he always did best. He was a truly abysmal military commander.