It's interesting to realise just how dated this book is upon re-reading it as an adult. I loved it in upper primary school, and it was still a fun re-read, but I found the family dynamics really hard to handle. Marcy's father is emotionally (and almost physically) abusive, and that's not really dealt with in a way that I found satisfying as an adult in the year 2013. Similarly, I'm not sure that her mother's journey rang true to me. Certainly, it would only be believable in a conservative religious setting in this day and age, but even in 1974, I think she may have read a bit as a stereotype from the past.
All that said, I think the book's greatest quality is its ability to show the psychological reasons for the person Marcy is. This is surprisingly subtle at times.