The most important Dominions skill

[Tichy]

I'm going to have to go with planning, too...particularly because it's what I have the most trouble with, and what seems to have the steepest learning curve.

(Maybe tied for first with battle scripting and troop placement) -- things like forging, thug equipping, pretender design, and early expansion strategy can be cribbed from posts and guides by more skilled players, but planning and the details of BF scripting can't, because that requires tactical and strategic thinking that can't be imitated.