Well, this is all very hypothetical stuff. There might be no downsides at all! Hell, if I could safely make my future children 6 inches taller and 20 IQ points smarter, I'd do it.

But many people have imagined dystopian consequences - creation of a lifelong underclass; negative side-effects that take decades to manifest; having big corporations and government gatekeeping conception; monoculture and loss of genetic diversity; what would become of us pre-editing humans; impact of state control of genetically-predicated personality traits; and so on.