Paul Fairchild, writing at The Atlantic:

For us mortals, “kryptonite” works without the cape and the big red “S.” It’s a moral weakness, a character flaw. It’s the idea that we’re powerless in the face of this vice or that guilty pleasure. It sounds cool when we describe our shortcomings this way, appropriating Superman’s virtue for ourselves: “cigarettes are my kryptonite.” This kryptonite is metaphorical, a weaker, abstracted copy of a space rock that serves as a totem. But it makes more sense as a metaphor than as an object that’s just a cheap, flimsy deus ex machina.

Everyone’s got their kryptonite. Everyone’s powerless in the face of something. Sex, drugs, money — those are high end character flaws. But just as Superman’s classic writers made “real” kryptonite into something bigger than it deserved, so metaphorical kryptonite turns out to be more deflationary. Our moral integrity is better undermined with things more mundane, such as catnaps or Friends reruns or jelly donuts. Mephisto delights. Screw the scheming, work, and showmanship that go into Faustian bargains.