commonplace vs old-hat

commonplace is a noun but old-hat is not a noun.

Word	Noun	Adjective	Verb	Adverb
commonplace	Yes	Yes	No	No
old-hat	No	Yes	No	No

As adjectives, commonplace and old-hat are synonyms defined as:

commonplace and old-hat: repeated too often; overfamiliar through overuse

Other synonyms of commonplace include banal, hackneyed, shopworn, stock, threadbare, timeworn, tired, trite, well-worn.

commonplace (noun)	old-hat (noun)
a trite or obvious remark

commonplace (adjective)	old-hat (adjective)
not challenging; dull and lacking excitement	out of fashion
repeated too often; overfamiliar through overuse	repeated too often; overfamiliar through overuse
completely ordinary and unremarkable