Besides replacing military generals and the success of the "blitzkrieg," what other factors helped Germany in the early years of World War II?

There are three reasons—apart from the use of blitzkrieg—that explain Germany's early successes in World War II (1939–1945).
Germany's non-aggression pact with the Soviet Union helped it achieve success early in World War II. During World War I, Germany had to fight on two fronts—against the Anglo-French in the west and the Russians in the east. The non-aggression pact helped Germany between 1939 and 1941. It enabled Germany to focus on its invasions of Poland and then France without having to worry about its flanks to the east.
Another reason for Germany's early successes was the faulty military strategy employed by the Allies. France's reliance on the Maginot Line was an especially bad strategy. In other words, the French approach was too defensive. France's military leadership mistakenly believed that WWII would be fought like WWI. In WWI, rapid offensive operations were rare.
And finally, the Allies suffered from poor leadership. This began change, however, when Winston Churchill became prime minister in 1940. He energized British resistance and held off the Germans. In 1941, both the Soviet Union and the United States joined Britain in the war.