I got my first feedback to this series last week from Faithful Reader Warren:
The thing that I always find fascinating about early Superman is that the villains are way more human than superhuman. The only one like that left anymore is Lex Luthor.
I've noted this as well, at least indirectly. Superman himself is more human back in the early years: he can't fly, he doesn't have special vision (like X-ray or heat), and while bullets will bounce off his skin, he's not invulnerable. (By contrast, on The Adventures of Superman radio program, he has been able to fly and see through solid objects right from the beginning.) Hence his foes have also been more human: gangsters, saboteurs, and various other ne'er-do-wells.
Superman's first recurring enemy in Action Comics was the Ultra-Humanite, a bald, mad-scientist type in a wheelchair: not only less than superhuman, but a damaged human at that. Ultra was designed to be the opposite of Superman; instead of a heroic strongman, he is a crippled criminal, albeit a superior intellect (which enabled him to escape death by transplanting his brain into another person's). Lex Luthor was introduced about a year later, and Ultra-Humanite was dropped from Superman's rogue's gallery, since Siegel and Shuster decided Superman didn't need two bald archenemies. Like the Ultra-Humanite, Luthor is a regular human being, though gifted with superior intellect, and motivated from time to time by world domination or revenge against Superman.
It wasn't until the late 1950s that Supe's more superhuman or non-human enemies began to show up in the comics, beginning with the Kryptonian android Brainiac in 1958, at the start of the "Silver Age" of comic books. Others soon followed, including the Kryptonite-powered Metallo and Bizarro Superman. (A notable exception is the magical supervillain Mister Mzyzptlk, who first appeared in the early 1940s.) At this time, Superman himself was also being written as an increasingly powerful hero. My personal theory is that this is a bit of a feedback loop: a more powerful Superman needs more powerful enemies to pose a credible challenge; conversely, more powerful enemies require greater powers to defeat. It's no wonder DC Comics tried to scale back Superman's abilities since the 1980s.
We, of course, have the ability just to travel back in time to a point where Golden Age Newbie Superman still punched out baddies. And so, without further ado, let us travel back to 1940, and a new Superman adventure . . .