WARNING: The following contains spoilers for Justice League #73, on sale now from DC Comics.
Under the influence of various forms of solar radiation, Kryptonians such as Superman have the potential to become some of the most powerful beings in the DC universe. As the Last Son of Krypton, DC's greatest hero possesses a few very unique weaknesses that can cut through his otherwise impregnable defenses to the point of endangering his life, and these weaknesses have been exploited to great effect by many of the Man of Steel's enemies
Although Superman's vulnerability to Kryptonite is well-documented within the DC universe, Justice League #73 (by Brian Michael Bendis, Emanuela Lupacchino, Szymon Kudranski, Wade von Grawbadger, Hi-Fi, and Josh Reed) has revealed that Man of Steel regrets revealing so much about his greatest weakness and has hidden his additional susceptibility to magical energies from the public. By admitting that revealing his lethal relationship with Kryptonite was a mistake, the Superman of DC's mainstream comic book universe has indirectly learned from his counterpart within the Christopher Reeves film series, whose decision to announce his weakness to the world nearly lost him his life.
After the Justice League failed to stop the primordial Lord of Chaos Xanadoth from possessing Black Adam and kidnapping the precognitive mystic Madame Xanadu, the team regroups within the Hall of Justice to plan their next move. When Naomi McDuffie, the League's newest member, suggests sending Superman after Xanadoth, the more experienced members of the team explain that Superman is exceptionally vulnerable to magic due to its ability to interfere with the Kryptonian biomatrix that makes him nigh-invulnerable under a yellow sun. After Naomi questions why Superman hid his weakness to magic from the public when he'd been so open about his weakness to Kryptonite, Detective Chimp explains that the Man of Steel deeply regrets advertising his vulnerability to the irradiated remains of his home world, referring to it as a "rookie mistake."
Superman's admission that his decision to reveal one of his only weaknesses was unwise is a direct nod to 1978's Superman: The Movie, which portrays a younger version of Superman who's still finding his footing as a hero. Hoping to gain the public's trust after his initial acts of heroism are met with fear and distrust, Superman engaged in a series of interviews with Lois Lane in which he revealed everything about his extraterrestrial origins, including the origin of Kryptonite and the toxic effect the element has on him. Although these interviews do help assuage the citizen's fear of Superman, they also give Lex Luthor valuable information about how to defeat the hero. During the film's climax, Luthor nearly kills Superman by dropping him into a pool filled with pieces of Kryptonite, with the Man of Steel only being saved after Luthor's assistant Eve turns against the villain.
The Christopher Reeves films aren't the only example of Superman paying the price for revealing his weakness to Kryptonite. Although he's found acceptance among the people of Earth, Superman has always longed to share his heritage with the people of his adopted home world, both to ensure that Earth can benefit from the advanced technological achievements of Krypton and to alleviate the deep loneliness that he feels as the destroyed planet's last survivor. Over the years, however, many of Superman's villains have found ways to weaponize Kryptonite and turn it against the Man of Steel, and their clever ruse of the element has nearly proven fatal against the hero on several occasions.
Although magic isn't quite as lethal to Superman as Kryptonite, its status as a fundamental force within the DC universe means that it's much easier to come by than the irradiated rock. In addition, most forms of magic lack the crippling side effects that direct use of Kryptonite has on human beings, making it a much safer option for anyone who wishes to harm the hero. In the hands of skilled users like Zatanna, magic could be just as dangerous to Superman as Kryptonite, justifying his decision to hide his weakness from the general population.
While Naomi may be hurt by Superman's decision to keep his weakness to magic a secret from her, it was a purely strategic move that reflects how much the veteran hero has grown. With the safety of the world and his family resting on his shoulders, the Man of Steel has learned that he can't always afford to be completely open and honest about every aspect of his life.