WordNet sense 1 (completely lacking in playfulness):

unplayful

WordNet sense 2 (acting with or showing thought and good sense):

thoughtful

WordNet sense 3 (dignified and somber in manner or character and committed to keeping promises):

sedate, sober, solemn, grave

WordNet sense 4 (characterized by a firm and humorless belief in the validity of your opinions):

earnest, sincere

WordNet sense 5 (appealing to the mind):

good

WordNet sense 6 (appealing to or using the intellect):

intellectual

WordNet sense 7 (of great significance or value):

important, of import

WordNet sense 8 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):

difficult, hard

WordNet sense 9 (causing fear or anxiety by threatening great harm):

dangerous, grievous, life-threatening, severe