Protestant culture...
...got over the edge while developing intellectual and rational part of a human's personality. People became too self-orientated and egoistic while searching for something they felt was missing inside of them. That's what is destroying The family as an institution in the western society, giving birth to every other social problem.
And that's what Joanna felt she was missing, too. Everybody's concentrating on pursuing their careers thinking that will make them feel complete but it only brings more misery.
*small spoilers ahead*
She was feeling lost, unfulfilled and not useful? Well, she could've got herself a job. Yes, mother is the most important person in every child's life (especially from 3-5 years of age), but she could've paid a nanny from the earned money. Ted worked, took care of a kid, without the nanny. All alone. He managed somehow. Because he loved his son and felt responsible for him. The child doesn't come from out of nowhere, by it's own will. Father and mother are responsible for that. where was Ted when his son hit his head? HE was there!
The bigger reason why Joanna left Ted was that she needed a man, meaning a lover. He neglected her cause of his work. But that need she felt was more important to her than her own child - that's where she lost every sympathy from me. It's simply selfish and irresponsible.