Harrow pretty much laid out a plan to kill all the D'lassios and Nucky stoped him because he doesn't want more bloodshed until after the elections.

The Commodore's comment about the wrong man running AC to Jimmy hints at where this show is going next season; Jimmy is going to start doing so much of the hard work for Nucky that it's going to become apparent who is more important in that relationship. I think what is more interesting in the show is how it's telling the development of three gangsters in America in the 20s: Jimmy, Al and Rothstein. Nucky is basically the story vehicle for Jimmy and Rothstein, and Jimmy is the vehicle for Al by way of Torrio; other than that, his romantic and political situations are the only thing he is really there for.