Noun vs. adjective: A nonprofessional may still give a professional performance, while an unprofessional person does not.
Webster’s New World College Dictionary provides a very helpful list of compound words formed with non- that do not have special meanings and thus “will be understood if ‘not’ is used before the meaning of the base word.” In case you were wondering, the dictionary also states that “a hyphen may be used after ‘non-‘ and is generally used when the base word begins with a capital letter. However, you’d better check with the applicable style guide to see if hyphens are preferred or not in the work you are reviewing.