WordNet sense 1 (the quality of wearisome constancy, routine, and lack of variety):

monotony, sameness

WordNet sense 2 (not challenging; dull and lacking excitement):

commonplace, prosaic, unglamorous, unglamourous

WordNet sense 3 (tediously repetitious or lacking in variety):

monotonous

WordNet sense 4 (lacking in liveliness or animation):

dull