What is wrong with designer babies/genetic engineering?

We do not know what the long term result of genetic manipulation will be. "Improving society" is a subjective term. Who is to decide what is an improvement? Should all people have blond hair and blue eyes? What if those attributes are from the same genetic package as "smart and physically fit as possible?" What if the combination of genes that produce the "desirable" attributes also deliver a tendency toward horrible disfigurement? The proponents of "genetic engineering" are not likely to give you guarantees.