slight vs unimportant

slight is a noun but unimportant is not a noun.

Word	Noun	Adjective	Verb	Adverb
slight	Yes	Yes	Yes	No
unimportant	No	Yes	No	No

slight (noun)	unimportant (noun)
a deliberate discourteous act (usually as an expression of anger or disapproval)

slight (adjective)	unimportant (adjective)
(quantifier used with mass nouns) small in quantity or degree; not much or almost none or (with `a') at least some	not important
lacking substance or significance	devoid of importance, meaning, or force
being of delicate or slender build

slight (verb)	unimportant (verb)
pay no attention to, disrespect