If you look at the history of music as a whole, rock is not ill defined. It's a pop music movement from the last 60 years, with a lot of similar characteristics. Of course, it sells itself as a myriad of subgenres, because part of it's idea as a musical movement and as a commercial product is to be very specifically tailored to it's target audience, but "Asking for Rock is only slightly better than 'Play me some songs that are music.'" it's not true, from 'Play me something done in the last 2200 years from anywhere on earth' to 'Play something done from 1960 to 2013 that identifies itself as some subgenre of the Rock family' there is an enormous difference.