Frame mounted firing pin (and the dreaded lock). On N frames, the frame doesn't have the "humped" appearance the J/K/L frames do to accommodate the lock.
One of the main benefits of this series in my book is that all except the model 19 have square butts, so original target and magna grips can be used. The altamont grips that come on the classics are nice looking, but are too thin and have a hard edge around the backstrap that makes them suboptimal, at least for my hands.