Is it learning what's on the page? or technical difficulty? or musical difficulty? or a combanation of all 3?
It’s the complex rhythms and unique tonal language. Also the very big chords, huge dynamic contrasts, incorporation of themes and the ability to be able to memorize his music. Vingt Regards prime example. Very few have performed it without score. I believe Steven Osbourne is one of them.
This guy performed them from memory:
It's a fantastic performance of exceptional music. He takes a very lyrical approach filled with musical sensitivity. Thunderous when it needs to be without being harsh, and whispering without loosing tonal support.
Messiaen's wife, Yvonne Loriod, played the work from memory.