One of the key mistakes I made, was to use the very precious Russian Armor to counter attack in Summer of 1942. Dice went south, and the Russians withdrew, to preserve their armor.
BECAUSE they retreated from battle, they were not eligible to move during the Summer Mech Move. This allowed the Germans to do all kinds of stupid things with their Mechanized units, without the threat of Russian Retaliation.
Also, if the Germans manage to take Moscow, the Russians can get it back, fairly easily. Lots of places to bring up new units.
If they manage to take Leningrad, boy howdy is it hard to recapture...