Details, Details
One of the hardest things to accomplish with digital painting is to know how much detail is “enough”. One of the comments I see frequently in digital painting forums is that the image “still looks like a photo”; while at the same time I’ve gotten comments that my work looks “fuzzy”.
The rule of thumb that I usually go by is that you have to establish a strong “focal point”, and to make sure that this element has more details than the background or other areas that are of less importance to the composition. That is the basic point of my “hybrid” technique as I use the second media (usually chalk) to emphasize details on the focal point. Of course the original composition of the photograph is of key importance also. An image that is too cluttered usually won’t be as aesthetically pleasing to the eye.
Ultimately, the most important factor is to show the viewer what you feel about the piece, and what you feel is important in that piece. And yes, it should look like a painting or a chalk piece. After all, they don’t call it “natural media” painting for nothing.