World War II was a total war. It involved the complete mobilization of the populations and the economic infrastructure of almost every nation involved. Civilians were required to make major material sacrifices, and often found themselves the subjects of attack. While civilian populations had always suffered during wartime, the Second World War saw noncombatants brought into the conflict as never before. In addition to the unprecedented brutality of Nazi Germany toward Jews, millions of Soviet, German, Japanese, Chinese, and other civilians perished during the course of the war, due to intentional targeting and massive aerial bombardments like that of England and Poland by the Germans and of Germany and Japan by the Allies.
The war also saw the use of new machinery which revolutionized the way it was fought. The importance of airplanes has already been mentioned, and indeed the German strategy of blitzkrieg marked their successful integration with mechanized armor with rapid, deadly effect. The war also saw the first use of sonar and radar to target weapons more effectively, and while the use of poison gas, a hallmark of the First World War, was banned, both sides used far more powerful explosives and other frightful devices like flamethrowers. Both jet engines and rockets were deployed for the first time in the later years of the war, and of course the war ended with the most horrific innovation of all, the atomic bomb.
I believe that paratroopers should be mentioned as a new aspect of war that was widely used in World War II. The Germans used paratroopers first. The Americans used paratroopers heavily in the invasion of Europe, although there was not much use of paratroopers in the Pacific theater. Through newsreels and documentary films, people have become accustomed to the sight of hundreds or thousands of men dropping out of planes with their parachutes blossoming like so many opening flowers or blowing dandelion seeds, but it was a new sight in World War II and foreshadowed the growing importance of air power in warfare.
Better technology meant more efficient killing machines, such as better tanks and aircraft. Although these were seen in WW1, now they were much more developed and more lethal.
Terror bombing of civillians to achieve shock purpose
Nuclear warfare, as US used in the end against Japan