There are a couple of moments where things could have gone very differently. Primarily by the Germans winning the war against Russia but the one that really sticks in my mind is Hitler not listening to Guderian in 41 about diverting the Panzer divisions of Army Group Center to assist in the liquidation of the Kiev pocket.
What it boils down to is Hitler was concerned about the speed with with German mobile formations were getting ahead of the infantry. He was also concerned about the size of the Russian forces trapped within the Kiev pocket. He ordered Guderian to take his panzer formations from where they were in army group center all the way to Kiev to assist in the assault on Kiev. Guderian argued that such a drive would already exacerbate the existing problems caused by the panzer formations being in continuous operation for months. The dire need for maintenance and refitting that was already causing breakdowns of large numbers of vehicles. Hitler overruled him, Guderian did what he was told, Kiev was taken along with a ridiculous number of prisoners, vehicles, and artillery. Then he took the tanks back up to the staging ground for the assault on Moscow. By then it was too late. They had to delay the attack to do the maintenance and refitting that Guderian had told Hitler would have to be done. That delay allowed the Russian fall rains to wreck the road network. Then the drive on Moscow began. With winter coming quickly, the effort seemed fraught with the risk of over-extending and leaving his command subject to counter-attack. He was ordered to proceed anyway. They made it to the outskirts then were forced back.
It's very likely had Hitler listened to Guderian and the other Army Group Center Generals, they could have taken Moscow and with that crippled Russia ability to fight the war. Moscow was a major center of arms production, the center of the Soviet communications system and an important transport hub, not to mention the crushing effect it would have had on Soviet morale. As it was they gave the Russians time to fortify and bring up veteran troops from Siberia to stiffen the defenses of Moscow.