Perfection. That's what we're striving towards. In principle, this isn't a bad thing, but it can lead to undue stress when applied to parenting. Society doesn't help us here, since it seems that parenting books tell us that following certain systems will result in perfect children and a perfectly happy family, while others tell us that whatever we're doing is wrong and will ruin our children. Now, there are some things that have been shown to be damaging, like CIO, but I think there are multiple ways of parenting, and you must find what works for you.
Interestingly, I've also heard this as a criticism of Attachment Parenting, with some saying it places too much pressure on parents to be absolutely perfect. I can see how some may think that due to some proponents of AP insisting that all parents must do certain things in a certain way, but I don't think it's a fault of AP principles in general.
Another problem I see with striving to be the perfect parent by the standards of society is that I find those standards/ideals to be unrealistic. When people ask you if your baby is "good", they generally mean to ask if the baby sleeps well and doesn't cry much, thereby not disturbing the parent very much. Is that realistic? I don't think so. When asked if my children are good, I answer in the affirmative, but I don't expect that they won't disrupt my life. They're meant to disrupt my life, in my opinion.
This stress on being a perfect parent also seems to ignore the fact that, no matter how many books or websites we've read, we're new at this. Even if it's our second or later child, we've never had that child before, and so we're still new at this. No child is exactly like another, and what works for one might not work with another. Therefore, even if you think you've figured out one child completely (which I don't imagine to be the case, seeing as people change and aren't robots), that person can't say they therefore have all children figured out. The bottom line is that we're going to make mistakes, and that's OK. We shouldn't place unrealistic standards upon ourselves. We're striving to be perfect, but it's a journey, and one for which we need God's grace.