With line art all colors become either black or white. There are no shades of gray. For some images this works well. When the appearance of shades of gray is necessary or to create special effects, convert the image to a halftone. In this type of B&W image black dots of different sizes, shapes, or spacing create the illusion of shades of gray - but only black ink is used.
When converting a color image to B&W you can often get better results if you first convert it to grayscale and then to line art or a halftone image. Additionally, deleting some elements makes the image look better once converted.
Although written specifically about scanning line art, this Halftone Screen and Line Art tutorial contains useful tips about using halftone screens for special effects.
