not bad vs great

not bad is not a noun while great is a noun.

Word	Noun	Adjective	Verb	Adverb
not bad	No	Yes	No	No
great	Yes	Yes	No	No

As adjectives, not bad and great are synonyms defined as:

Other synonyms of not bad include bang-up, bully, corking, cracking, dandy, groovy, keen, neat, nifty, old, peachy, slap-up, smashing, swell.

not bad (noun)	great (noun)
	a person who has achieved distinction and honor in some field

not bad (adjective)	great (adjective)
excellent	excellent
	in an advanced stage of pregnancy
	uppercase
	remarkable or out of the ordinary in degree or magnitude or effect
	of major significance or importance
	relatively large in size or number or extent; larger than others of its kind