Much has been made of the German delay in the summer of 1941 and how it supposedly was instrumental in thefailure to take Moscow before Winter came. Utter nonsense. There was no unplanned delay due to other operations. First, the Germans had to wait for the ground to dry out along major portions of their proposed axis of advance due to the usual heavy spring rains in those areas. They knew this and planned for it. Second and third, and much more impactful, were German failures in intelligence and logistics. Germany did not have a detailed Order of Battle for the Soviet military in 1941, thus they had no idea of what the Soviet military was capable of, especially in terms of absorbing casualties. Time and time again the Soviet Army was declared kaput due to heavy losses only for the Germans to see the losses replaced and even new formations raised. The Soviet manpower pool was significantly deeper than Germany, thus they could fight a war of attrition and win (this above anything else, was why Germany lost the war in the East). Nor did the Germans have much detail as to Soviet industrial capacity. If you don't know how much your enemy can produce, you have no idea when you will, or even if you can, break him. The Germans went into Russia on June 22, 1941 with virtually no detailed maps of the proposed battlegrounds. German units very often had nothing more than tourist road maps at the point of the advance. What looked to be a major highway on a map often turned out to be a dirt track. Imagine trying to push an armored division down a highway that turned out to be a dirt road. That caused huge delays as new routes of march had to be reconnoitered and, in fact, entire Corps had to shift their routes of march. The Germans also had a very limted supply system. The German army in 1941, contrary to popular belief, contained very little motor transport. The bulk of German transport was horse-drawn (even up to the end of the war). This caused a fundamental problem: armored units often waited on horse-drawn support units to catch up violating one of the fundamentals of war-when you get an enemy on his heels do not let up lest he gets time to re-organize and dig in. During the advance in the early stages of barbarossa, German panzer divisions sat still for days waiting for infantry to catch up and provide secure flanks. Much has also been made of the attack on Greece and Balkans as a reason for the delay. Again, little truth to that. It didn't matter if those units would have been available sooner, since much of the Western regions of the USSR were a morass of mud until June.
A lot of erroneous opinions as to why the Germans failed have developed through the years, due in large part in my opinion, to shows on the History and Military channels. These 30 or 60 minute TV shows should not be considered as reliable sources since their focus is to entertain not educate. Entertainment seeks short, easy to digest blips for mass consumption to explain what are often complex or arcane circumstances. I'll give a ready example: Watch a TV show on the Batlte of Stalingrad. More likley than not a reference will be made to "Hitler's obssession with Stalingrad because it bore Stalin's name" (I've actaully seen/heard that on the History Channel). Complete and utter nonsense. Hitler, and the German General Staff for that matter, wanted to take Stalingrad to protect the flank of their adavance towards Maikop and, more importanlty to cut the Volga River and prevent the Soviets from using it to move men and supplies. 70% of all Lend-Lease aid to the Soviet Union came through Persia and into Russia via the Volga. Cutting the Volga would have severly hampered the Soviet Union's logistical system; possibly a fatal blow. Stalin and his Genrals knew this of course and that is why they defended Stalingrad fiercely.
Lastly, the other major reason the Soviets defeated the Germans was Hitler's constant meddling in levels of detail that should have been left up to the experts. Stalin made similar errors in 1941 but he learned from his mistakes and turned over the day-to-day affairs of the Soviet military to his Generals. They in turn, won the war for him.