When considering your use of line, also take into account a) how its character relates to your subject matter-- either how it best serves the expression of your subject's form, or expresses the temperament you wish to convey and b) how the line is born of the medium you are using-- different drawing media are capable of producing different lines.
As far as different line quality or character is concerned, consider the following types: mechanical, structural, lyrical, constricted or aggressive, handwritten or calligraphic, implied, blurred and whimsical.
Here are some student examples of different line quality resulting from considering the use of different drawing media, as it records the same subject:
1. Boot Studies
2. Fruit Basket Studies
3. Spine Studies
4. Boar's Head Studies
5. Shoe Studies