I wrote recently about how some writers don't understand Superman's true weaknesses, but I think there is another flaw in the Superman mythos—just not in the traditional sense. The flaw is in his villains. Who do we have: beings like Lex Luthor, Metallo, Zod, Darkseid, Brainiac and Bizarro?
While some act as a mirror to the hero—and villains work best as mirrors and foils—they do so in comical ways. Their motivations are greed, hate, revenge and a lust for power and control. Who cares. They even go after Superman in less-than-creative ways.
(Side-note: perhaps the single exception is Mr. Mxyzptlk, who began with cartoonish designs on conquering the world merely because of a whim. Those devolved into just tormenting Superman for fun. But Alan Moore changed that with the Whatever Happened to the Man of Tomorrow? Storyline. Here, a childish naiveté has him experimenting with a spectrum of moralities until he lands on nightmarish evil, which manifests in truly interesting ways. )
Batman, on the other hand, has a more interesting gallery of villains to explore: Joker, Two-Face, The Penguin, The Riddler, Hush, Scarecrow, and Bane. These guys explore humanity and Batman through their very existence. Many of them exhibit stories of self-inflicted tragedy-turned-insanity, and offer interesting studies into the dark side of humanity. There are arguments that few of them have the kind of clinical insanity that would qualify for a legal defense, but they glow with a rainbow of psychosis, obsessive compulsions, and dissociative identity disorder. (Two-Face rarely switches personality, but he is obsessive about letting a coin make decisions between good and evil—at times, he slips into a truly dissociative state. The Riddler struggles to not leave a riddle, but he cannot, even though he knows it always turns self-defeating.) And all of these typically call back to some tragedy.
Of themselves, they can be mildly interesting, but the real interest is what they say about the Dark Knight himself. There are moments where Batman notes the similarity between himself and his villains. Batman is loaded with psychological questions about PTSD, obsession and delusion. He has noted that he didn't become Batman because of his parent's murders, but that the event shaped him into the hero rather than yet-another Gotham villain—but the seeds of crazy were there simply because that is what Gotham does to people. Deep down, everyone there is crazy. Arkham Asylum is just a magnifying glass for what the city itself offers. And some writers have even suggested that Metropolis and Gotham are just caricatures of New York City's awe-inspiring and ugly sides—so even the settings reflect the characters.
Where Superman is a boy scout whose villains press him to find creative ways to avoid moral gray areas, they are often flat cutout characters like the Man of Steel himself. Batman is a traumatized, obsessive-compulsive anti-hero whose villains reflect his obsessive, delusional, dark path.
Writer of speculative fiction. Current work in progress is an untitled dark fantasy novella.