I know I don't like to see visible inclusions. Consumers want to know how high up they have to go in terms of clarity. I am not sure about grain lines. In my limited understanding the lines they may have little effect. If oriented wrong I think it could channel light in the wrong direction.
Fractures are less desirable.
I think they could reflect light away from the viewer. To each their own, but my wife's major diamonds are SI1, no fractures, feathers, or cleavages. Cut - by far the most important 2. Most people would not agree with my putting size before clarity, so let me explain. For me clarity is the least important factor as long as it meets several minimum requirements - this is just my personal opinion.