On the left, the lines were done in more sweeping strokes. They are more confident.
On the right, however, there are lots of little broken lines. The contour of the image gets broken up and it creates a "messy" look that looks very amateur-ish.
People often start with their drawings looking more like the right side since they don't know what they're doing. When you sketch, challenge yourself to use only a few long strokes and still get the message across. ^^ You'll be surprised about how much information can be conveyed with just a few lines on a page.
