Oz was groundbreaking in a number of ways, most obviously in its visual impact. Movies in color had been made for a while, but most films in 1939 were still in black and white, so the gimmick of beginning in B&W and shifting to Technicolor was very effective. Some of the special effects were advanced at the time (and are still one of the movie's strengths). One of the most famous sequences, the tornado which sweeps across the farm fields, was created by filming a windsock being blown around by electric fans. It's more realistic and believable than the computer-generated tornadoes in the movie Twister, made 57 years later. That's progress.
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