The bad points of a one child policy is : Lazy , bad manners ,dilatory ,pay no attention to hygiene and so on.
I disagree with Xiang above. None of that has anything to do with being an only child and mostly are inconsequential anyway. As the large amount of only children in the US and other countries will prove. I am basically an only child, not particularly bad manners, not lazy, and I have good hygiene and pay attention.... The worst things in my opinion are (in no particular order): Selfishness, inequality of numbers between the sexes, if your child dies, lack of socialization with the opposite sex/same age group, the killing of daughters because a couple want a son and many others come to mind that are far beyond the inconsequential.