great vs not bad

great is a noun but not bad is not a noun.

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

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

Other synonyms of great include bang-up, bully, corking, cracking, dandy, groovy, keen, neat, nifty, old, peachy, slap-up, smashing, swell.

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

great (adjective)	not bad (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