The Superman cartoons from the 1940s were animated with a beauty and fluidity not seen before, but they were also hugely influential outside the world of animation.
In order to achieve realistic movement, the series' animator Max Fleischer -- who also created Popeye and Betty Boop -- used a technique he invented called rotoscoping. The process involved animating on top of film footage, and has been used since in everything from Star Wars to Waking Life. This was largely possible thanks to the ludicrously high budget he and his studio requested from Paramount (about $US1.6 ($2) million per 10 minute short, adjusted for inflation). But just look at how life-like the results are. Don't feel weird about watching it -- the episodes are all public domain.
Monsters in the series, like a gigantic, city-destroying lizard, went on to influence works like Godzilla, and the looming, art deco style of Metropolis would predict the coming trend of film noir and influence the aesthetic of the much beloved Batman: The Animated Series.
The cartoon was also the first that gave Superman the power to fly. Previously he had only been able to leap tall buildings in a single bound.
As kaptainkristian points out, not only is Superman a towering achievement of Golden Age animation, but it also teaches an important lesson that too few works have learned from since: origin stories are boooooooooooring.