effortfulness - the quality of requiring deliberate effort

ruggedness, hardness - the quality of being difficult to do; "he assigned a series of problems of increasing hardness"; "the ruggedness of his exams caused half the class to fail"

subtlety, niceness - the quality of being difficult to detect or analyze; "you had to admire the subtlety of the distinctions he drew"

quality - an essential and distinguishing attribute of something or someone; "the quality of mercy is not strained"--Shakespeare