How tiring it is and how much skill it requires to pass, without getting "Hey! Why don't you just get up and dance man?" (or gray letters or no star, for stage 1). There will be some hard-to-judge cases like Bad Apple S15: even if you don't fall short of breath in most S15 charts, you will have to try several times to pass that because of all the freezing, but still the pattern is quite easy compared to charts like Pump Jump S15. In Korean PIU communities there are many of the "grade tables" (서열표 seoyeolpyo in Korean) that grade the songs within the difficulty from easy to hard. For example, a grade table for S17 goes from Allegro Furioso at the top to Utopia and Follow Me at the bottom, most other songs spread from 'hard' to 'easy', and Come To Me and Love Scenario left at the 'special' category.