playfulness vs serious

playfulness is a noun but serious is not a noun.

Word	Noun	Adjective	Verb	Adverb
playfulness	Yes	No	No	No
serious	No	Yes	No	No

playfulness (noun)	serious (noun)
activities that are enjoyable or amusing
a disposition to find (or make) causes for amusement
a festive merry feeling

playfulness (adjective)	serious (adjective)
	concerned with work or important matters rather than play or trivialities
	completely lacking in playfulness
	causing fear or anxiety by threatening great harm
	requiring effort or concentration; complex and not easy to answer or solve
	of great consequence
	appealing to the mind