The anatomy thing, I won't comment on. But c'mon guys let's not be selective with our standards. srsly. You can't hold some artists to a standard and excuse others.
I would suggest doing some digital paintings. Pick out some photographs of simple, solid objects with strong light and do a speedpaint of it. Use a hard brush on 100% opacity. Don't worry about blending, just worry about getting the light shapes and shadow shapes.
Note the word "shapes". Where you're failing here is you're not understanding that lighting isn't always this soft gradient of shadow on round objects. Light and shadow both form strong shapes depending on the surface and direction of light.
Observe the work of Craig Mullins, Jamie Jones and other concept artists. They don't really ever use lines. Not that you need to stop using lines or anything, but you'll know your shading is good when you can convey an object with just values (shadows/lights).