I will add a couple of points I learned from my history teacher son:

1) There was a many months delay on the start of the operation. This resulted in the troops arriving in the east under much colder conditions than had been planned for; and

2) By 1941 the third reich had massive corruption (not dissimilar to what is happening in the US with private weapons contractors being nearly completely integrated into the governent). The equipment, particularly the soldiers' kit such as boots, were of extremely bad quality. It is a long way to the Eastern Front from Germany, and gear did not hold up.

Both of these points add to your main point that by the USSR not folding immediately, but instead slowing down, retreating and giving land up (they have plenty) they turned the war from a sprint into a marathon.