I have heard it was FDR's worsening health that led to his surrendering ground to Stalin and that, in the end, FDR knew exactly what sort of person he was dealing with.
The one point no one has mentioned is China. FDR was a supporter of Chiang and I don't see him turning on his ally after what she had suffered at the hands of Japan so at worst there is a divided China, at best Mao is crushed. Either way, down the line there is no Korean or Vietnam War which will hurt Japan somewhat (the Korean War brought in a large influx of American cash). This might slow Japan's rise with the repercussions that brings.
As to who follows FDR, I think voter fatigue with the Dems coupled with the economics following the war will see a Republican in office. It won't be Eisenhower (he only ran to stop MacArthur) so it's very likely Dewey.
Another change will be America's military. It was already demobilizing and returning to it's former stance of isolationism centering it's new forces around it's Navy and Air Force. Without Korea, which showed the shortcomings of post-war American military strength, we might not see the growth of the military industrial complex nor America's pro-interventional stance.
I don't see much of an immediate change with Churchill's early death. Atlee becomes PM earlier but carries on as he did in OTL. What is interesting is what happens in the 50s. Will America standby during Suez? What about Iran?