What defines a hard piano piece anyway? Just by listening to the piece, it seems very hard and complicated...his score looks very complicated at a first glance as well, but if you break it down, its not as bad as it looks. Its just long. Dont get me wrong, its very scary looking and difficult...but still, the hardest? By browsing through his score, I can see that he doesn't really follow too many of the traditional rules, and he uses alot of 16th, 32, and maybe some 64th notes. Also, there are alot of irregular note groups....However the left hand for the most part is in sync with right hand, despite his irregular patterns. (the right hand notes divided by the left hand notes = a whole number) So even his irregular note patterns arent THAT hard to play. I am pretty sure there are alot of very hard piano pieces out there, that may be just as hard, and be very regular compared to his concerto composition. In my opinion, I think the readability, and the way he scored his notes make people think his concerto is the hardest.