I don't like the black lines, they kill the look for me. I rarely see fully rendered illustrations or concept art with heavy dark lines. I like Valve's solution for a painterly/illustrative look without being a toon-render. Use rim lighting instead of black lines!
In general, the texture maps used on the 3D world are impressionistic, meaning that they are painterly and maintain a minimum level of visual noise. This is consistent with the style of painting used on background plates in many animated films, particularly those of Hayao Miyazaki, in which broad brush strokes appear in perspective, as if present in the 3D world rather than on the 2D image plane [Miyazaki 2002].
In the early stages of development, many of these 2D textures were physically painted on canvas with watercolors and scanned to make texture maps. As we refined the art style of the game, texture artists shifted to using photorealistic reference images with a series of filters and digital brush strokes applied to achieve the desired look of a physically painted texture.
This last expression is merely the per-pixel normal
dotted with the up vector, clamped to be positive. This causes the
dedicated rim lighting term to appear to add in indirect light from
the environment, but only for upward facing normals. This is both
an aesthetic choice and a perceptual decision designed exploit the
human instinct to assume that lighting tends to come from above.
"Astronomy offers an aesthetic indulgence not duplicated in any other field. This is not an academic or hypothetical attraction and should require no apologies, for the beauty to be found in the skies has been universally appreciated for unrecorded centuries."