For me, personally, the 'Isle of Joy' is much much more difficult than the "Waldstein." And, yet - there are some comparisons in the freedom of movement and expression in the middle of the waldstein and throughout the isle of joy.
The first difficulty I had with the L'Isle was counting out the beats - so finally in the first measures I put in lines slashing through treble/bass that counted out '1-2-3e&a4e&a' The first group of thirty-second note/s would come on beat 3. The second on '&,' the third on 4, the fourth on '&' again. You can subdivide from there every other thirty-second to be 'e' or 'a' in the pattern 1 e & a ...
Then, you just throw it all up in the air after so much practice with the timing and play 'Quasi una cadenza.' For some reason, I kept wanting to maintain precision. This is my classical side coming out. I don't want to make a mistake with timing. Therefore, i am worried about these little notes and can't relax fully. I think for a while - i hated Debussy (not personally, just what he wrote).
Debussy also requires you to think 'out of the box' in MANY areas. Beethoven stays pretty close to the natural limits of pianists in the waldstein - but Debussy kind of says ' imagine yourself as an alien.' He can change beats and rhythms suddenly and this rapid change makes you have to always be thinking ahead since there are many notes per measure/page.
Beethoven did make some unusual key modulations and changes in the Waldstein - and so does Debussy. Look at around measure 114 at the key change from three sharps to C major - but plays all B's to start with the idea that he is actually in limbo here - musically speaking and adds in a C# soon to make things even more confusing. As i see it - the only thing connecting him and Beethoven is the use of the V chord and trilling because even Beethoven is somewhat more sensical about his diversions. Debussy is this way throughout. The instability is made to feel stable. In Beethoven you always know when youare going through a period of instability. But, of course, one could argue that even the beginning of the Waldstien is very unstable and unpredictable.
PS the trills I'll leave for someone else to talk about. I easily played the waldstein trills but somehow had more trouble with the ones in Debussy. Perhaps because there seems to be more of them - and you really have to have a lot of strength in you hands and not overdo it and kill your hand when playing the trill with a third. It's an unnatural position imo. When you have a simple trill - such as at the beginning - your thumb doesn't play into it at all. As soon as you lift your thumb to play the third below - it slightly changes the position of the hand.
Topic: Debussy - l'isle joyeuse