all-important vs essential

all-important is not a noun while essential is a noun.

Word	Noun	Adjective	Verb	Adverb
all-important	No	Yes	No	No
essential	Yes	Yes	No	No

As adjectives, all-important and essential are synonyms defined as:

Other synonyms of all-important include all important, crucial, of the essence.

all-important (noun)	essential (noun)
	anything indispensable

all-important (adjective)	essential (adjective)
of the greatest importance	basic and fundamental
	defining rights and duties as opposed to giving the rules by which rights and duties are established
	being or relating to or containing the essence of a plant etc
	of the greatest importance
	absolutely necessary; vitally necessary