nastiness vs unpleasantness

nastiness and unpleasantness both are nouns.

Word	Noun	Adjective	Verb	Adverb
nastiness	Yes	No	No	No
unpleasantness	Yes	No	No	No

As nouns, unpleasantness is a hypernym of nastiness; that is, unpleasantness is a word with a broader meaning than nastiness:

nastiness (noun)	unpleasantness (noun)
the quality of being highly unpleasant	the quality of giving displeasure
malevolence by virtue of being malicious or spiteful or nasty	the feeling caused by disagreeable stimuli; one pole of a continuum of states of feeling
a state characterized by foul or disgusting dirt and refuse