keen vs not bad

keen is a noun but not bad is not a noun.

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

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

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

keen (noun)	not bad (noun)
a funeral lament sung with loud wailing

keen (adjective)	not bad (adjective)
excellent	excellent
intense or sharp
having or demonstrating ability to recognize or draw fine distinctions
having a sharp cutting edge or point
painful as if caused by a sharp instrument

keen (verb)	not bad (verb)
express grief verbally