I think what separates the "sheep" from the "goats" are a few things not mentioned in either the codebooks or the trade classes.
The first is having an education that includes at least the basics of other trades.
The second is having the ability to imagine a room in use by the customer- and designing accordingly. This can include needing some "psychic" abilities, in anticipating the customer's use.
Finally- and this may seem obvious- not leaving the juob, or saying you're done, until you are certain everything is working right.
