The A section is a wonderful and majestic melody, but Schubert has planted the diminished fifth aka the tritone, the forbidden interval in church music like little mines throughout. Its just the composer being naughty.
At 3:52 the B section turns to the minor, and the melody is suspiciously like the Dies Irae from the requiem mass.
At 7:56, the A section returns in drastically shortened form.
At 8:47, the C section is a long fugue. Schubert was taking counterpoint lessons from Simon Sechter, and this is a good example of how fugues began popping up like mushrooms in Schuberts output in his last year. The tritone is the glue that holds the fugue together, and it pops up everywhere.