What is notable about the structure of Schubert's final piano sonata?

The D. 960 sonata has four movements: Molto moderato, Andante sostenuto, Scherzo: Allegro vivace con delicatezza, and Allegro, ma non troppo. The opening Molto moderato is one of the most expansive first movements in the piano sonata literature — Schubert knew this was his last word in the form, and he made it count.