I just purchased Gregg Kreutz "Oil Painting Essentials" over the weekend. I just read the first page but already learned something. This is what he said. "after forty years of intensive research, I've figured out exactly what these prime essentials are:(for a great painting)
Accuracy: Make sure you accurately depict the subject.
Design: Arrange the material in a dynamic pattern.
Depth: Make sure the near/far feeling of space is convincing.
Drama: Intensify the visual energy.
The painting has to run on all four cylinders. (his words)
He said that some people may be good at one or two of the above but it must have all four.
I have his other book "Problem Solving for Oil Painters" which is very helpful too.