Breastfeeding gives the word "fashion" a whole new meaning. All of a sudden, you are looking for bras, tops, and dresses that have easy access to your nipple. And although it may be easy to find out what to wear while nursing, you may be hard press to find information on the bras you should avoid while breastfeeding that have nothing to do with the aesthetic or style of the bra.
Any woman who has breastfed for any length of time can tell you that breastfeeding boobs and non-breastfeeding boobs are two vastly different sides of the same coin (or, woman.) Breastfeeding boobs, in a word, are unpredictable — sometimes they leak, sometimes they're hard as a rock, sometimes they feel a lot like pre-nursing boobs. But nursing moms will also tell you that the proper nursing bra can make the difference between boob jail and boob semi-liberation. Since not wearing a bra when you're breastfeeding (especially at first,) is risky business, finding bras that fits well and isn't too constricting should be high on your priority list.
Knowing the types of bras not to buy is a good starting place for jumping into the vast yet unfortunately limited world of nursing-friendly bras.