I fail to see the purpose of the flashbacks to Nucky's childhood. The character is fully developed over the last 4 years and most of us could have imagined his background without going into lengthy underdog exposition. Does not add more depth to the character and I doubt we will get a retro-plot-twist.
The flashbacks could have been easily replaced by more 1931 background setting, other character screen-time, etc.
All in all, wonderful series!