Jason created a video about line quality and its various uses. He wasn't planning to do this, it was in response to discussions among level up members - I think that's awesome teaching technique. Notes follow:
Lines pulsate - get lost and found due to atmospheric perspective, overlapping, emphasis etc.
Line can push and pull form - fade to pull it forward into light front plane, or push it back into darker back plane by thickening and darkening.
^ Can also do the opposite, pulling forward with dark/thick, depending on what strategy you're using on that part of the drawing.
This all works with light lines too, not just dark lines.
Crisper line closer to viewer, making form pop, line fades as it moves into background planes.
Can also layer values to layer planes - dark forground & background with light mid, etc.
It isn't always about light/shadow, line can also be used to clarify form where needed.
Line doesn't necessarily mean a single mark, it can be a conglomoration of marks or a light line can be an area left by the absence of marks or made by eraser or paint etc.
Often there are thick lines on outer contour of a figure or form, thinner lighter lines describing the interior of it.
Lines can break up to 'breathe', to let some air and light in.
Lines can be used to describe space (depth, environment), form and also light.
Line can also be used to stress weight, movement and tension.
Less detail, lighter, thinner lines on moving parts - suggests motion blur.
Darker, heavier, crisper line describes something still and clearly seen.
Can show stress, the active muscles and tendons are more stressed so they pop more.
Michelangelo trick - accented rhytmic diagonals. A diagonal that's carried across from one side of the form to the other, where it disappears behind the form it darkens briefly, and also where it re-emerges on the other side, from around behind the body.
Used to create focus (emphasis).
Even with multiple drawings on a page, there can be primarly, secondary, tertiary areas of interest (focus, emophasis). Done with line as well as tone, degree of information etc.