The thing is, you have to look after that, to the conferences of Yalta and Potsdam, which were close to the end of WWII in Europe and after WWII in Europe, respectively. At Yalta, Roosevelt tried to keep the peace between the Allies, and was very kind to everyone, including Stalin. Churchill also did the same. In this sense, the Americans did do something important. However, when Truman came along, he messed up badly, stating that the USA had a super weapon. Stalin already knew because of his espionage, so did not make that much of a reaction, and he also thought the Americans would never use such a horrific weapon. Needless to say, he was wrong.
But back to the war. The Americans only entered at the end of both wars, but them entering is the only reason the Allied Powers won (arguably). At both times, supplies were running low and tensions were running high, so a new powerful ally did help to win the war. In the case of Berlin, yes, the USA did not do much, it was mostly the USSR. And where you said that the USA could have taken over Japan quickly, I beg to differ. The Japanese would never surrender, they would always fight to the death. Therefore, there were many discussions as to what should be done. If they had fought straight on, many more Americans would have died. Plus, the Americans had to island hop from Hawaii to Japan, quite a large distance.
But I see your point, and I would conclude that the USA helped the war effort indirectly by providing supplies and such in Europe, but handled Japan "exclusively" (The USSR was bound to help by the Potsdam Conference).