I'd say that if somebody wants to seriously rate difficulty, they would be best off with categories. Some pieces are obviously more difficult than others-for example, the Hammerklavier is more difficult than Sonata Pathetique by basically any standard. However, for those pieces that are similar, categories would be a useful way of rating difficulty-the pieces within each category would pose different challenges to different pianists. For example, perhaps the categories could be: beginner (simple melodies, simple movements), easy (things like fur elise), intermediate (things such as a number of Chopin preludes, a number of Scarlatti sonatas, etc.), difficult (many Beethoven Sonatas, Chopin etudes, etc.), advanced (harder sonatas by Beethoven, Liszt, Prokofiev, easier concertos, difficult toccatas, and the like), and expert (Things like Hammerklavier, Gaspard, Rach 3, etc.). Finally, if you wanted it, there could be a category called extreme (i.e., Sorabji, Xenakis, Boulez, and many other twentieth century composers who created works that are playable, but only barely).