I think Sea Tyger got most of it
When it came down to it, it was a combination of factors:
American industrial muscle - both the Japanese and the Germans thought America would take far longer to shift into a war economy before fighting back. Their big mistake was that America didn't need to be in a war economy.
On a related note, Roosevelt's determination to be on the Allied side even before Pearl Harbor, in spite of Congress. I think the USA would have been in before long, regardless. Churchill wasn't above editing intelligence to manipulate his allies!
The determination of a number of peoples to avoid being conquered, not just the Brits and Russians. The French, Polish and Czech undergrounds kept a lot of troops tied up who might otherwise have been in battle.
The SS/Nazi obsession with racial purity. A huge number of elite troops and resources were involved in the Final Program and with rounding up the victims. A number of German intellectuals were Jewish, and were driven out by the regime before the war.
The lack of resources for the Axis powers - Japan went to war to expand her sphere of influence, Pearl was supposed to disable the US Navy long enough for her to exploit the new territories, while Germany's attacks on Greece and Russia were both motivated by the need for oil, rubber and other industrial resources. My A-level History teacher pointed out that the Nazi economy would have consumed itself if Hitler's empire did not keep expanding.
I don't think you can blame Hitler's leadership problems on a disease - he just wasn't a very talented coordinator. All of his plans were created by underlings, but because he ran his government on a competitive basis (different departments competed for the same resources for the same jobs), which plan got implemented was dependent on who was being nice to Bormann at any given moment. Hitler was prone to responding favourably to whoever told him what he wanted to hear.
TBH First, the German nuclear plan was never anywhere near completion - and they had no way to deliver one beyond Britain anyway. Most of the best scientists had already fled the increasing discrimination before the war and Nils Bohr was utterly convinced by some bad data he received early on - as far as he was concerned, Germany needed alot more uranium to set off a nuclear chain reaction than was practical. This was the primary hold-up, as it led to continual holding back of funds.
This should be fun, well done Eric!
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)