I was under the impression that after disasters at the beginning Stalin realised he was not a good strategist and allowed his generals freedom of action, something that Hitler failed to do.
Soviet troops were motivated by a combination of ~Corsair#01~ and Strategos arguements. Fear was a big part of course but fanatical loyalty was as well. WW2, or the Great Patriotic War, to give it its correct title

was massively important to the soviet regime. It gave the communist government legitmacy, and allowed them to tap into russians feelings of nationalism, which was particaully effective due to the feeling of national unity caused by the German invasion. This, combined with the popularity with the regime among certain groups in society, such as the young and workers, which existed in the 30's and can be seen in the collectivisation programme and 5 year plans, meant that during and immediatly after the war the regime was actually popular. The war made the internationalist communism, which had been heavily eroded already, into a much more national communism, (a good example of this is the change of anthems in 1942 from the communist international to the anthem of the soviet union).
I believe that Stalin was also very important to the soviet war effort due to the economic 'progress' made in the 1930s. The five year plans were massively costly, in economic and human terms, inefficient and wastefull. However, they did propell the Soviet Unions economey into 2nd most powerfull in the world. and also crucially huge new industries were built from scratch behind the Ural mountains rather than in western Russia, meaning that the German invasion did not damage the SU industry as it would have previously done. The alternatives to Stalinist economic development which were being put forward in the 1920's by different factions in the Party would not have resulted in the same economic development. Bukharin's gradualist policy of development involved 'rid(ing) into socialism on a peasant nag', not the kind of breakneck industrialisation required for the worst conflict in human history.
Stalinism also created a brutal, centralised, disciplined society where public opinion had no sway, this was usefull for the war as I dont think democracies could have taken the same level of punishment, destruction and death without popular unrest or collapse.
Stalin had, however, serious weakend the red army in the 1930's with purges and this massively hampered the USSR war effort in the beginning.