Vallée is a long piece, with multiple sections. Each section requires a technical aspect that you need to exploit. I think the octave passages are probably the hardest. Even the "slower" more lyrical sections require you to be able to voice your melody above the accompanying parts. Similarly the huge chordal section needs you to be able to voice your melody above the chords, not to be "buried". All-in-all, this piece does contain all the "Lisztian" technical display. Lastly, the many sections need to flow and not sound disjointed. To be able to do that, one has to be very comfortable with each section and to be able to pull together the whole piece so that, as you said it, sounds "easy"!