For the sake of example, these illustrations all have distinctive rough lines on their composition:
post #2722765;
post #2723642;
post #2722641;
post #2722424;
post #2722464;
post #2722486;
post #2722511;
post #2722386;
post #2722323;
post #2722334.
(These posts are not considered sketches.)
A "rough" tag should exist, used when the illustration is not a sketch but has rough lines, visible individual strokes for shading, outlines intermingled with each other when they really shouldn't (for example, right sleeve on post #2722386), shading by scribbled lines (for example, neck on post #2722334), outlines that stretch further than they should, colors that go past their outlines (even if a matter of a few pixels).
Ideally, there should also be a "rough_background" tag for when only the background of an illustration has these traits. For example:
post #2722260;
post #2722246.
The usefulness of this tag would include:
Artists looking for reference. Rough lines and shading are evidences of how the picture was made;
Quality filtering. Rough illustrations can be considered to be low quality and increase the browsing experience when filtered out.
(EDIT: Let me rephrase the above statement. Rough illustrations can be considered to be low quality subjectively, and it would be good for the browsing experience if they could be filtered out.)
Updated