I assume I have a limited knowledge of WWII history on that front, but from what I was taught in history class, the biggest deterrent of the Nazi invasion of the USSR was the weather. What we learned was that the Russians were invading Germany more than the Germans could invade Russia. Basically the Russians weren't fighting to keep the Nazis from invading, but were focused on a counter-invasion.
I could very well be wrong, as I took WWII history in the US (which tends not to focus on European involvement in WWII).
Weather helped somewhat to slow blitzkrieg down in 1941 (though it applies to both sides equally; Soviets were just better prepared), but it doesn't win wars. If you look at casualty figures, 2/3 of all Axis casualties in WW2 (this includes Japanese!) happened on the Eastern Front in Europe; and most of those weren't frostbite.
Soviets paid two for one in terms of casualties, though. But they could keep doing that for longer, and their industry could keep up as well (war production figures for Reich vs USSR are rather telling).
I could very well be wrong, as I took WWII history in the US (which tends not to focus on European involvement in WWII).