Is introvert a derogatory term, or just neutral?

Solution 1:

Introvert and extrovert are terms used in psychology and study of personality types. It's not derogatory, though our society tends to value extroversion over introversion. Silly us.

Solution 2:

A long-winded discussion on introversion and extraversion is way beyond the scope of this website; however, generally speaking, introverted people tend to avoid big crowds and enjoy solitary activities.

The word in itself has no explicit negative connotation associated with it, as it's simply considered a type of personality by professionals like psychologists. That being said, however, in especially Western societies, introversion is often stigmatized and stereotyped by others, such as in the media and is generally (incorrectly) considered undesirable.