1) The new Architect has debugging turned off by default. I needed to turn it back on.
2) My class Overrides were not migrated properly during the upgrade, so (now visible) errors were getting thrown about colliding requires.
3) New Sencha versions were introduced that were also not migrated in my app.html file.
4) Some very odd issue (still unresolved) with "Ext.setup has already been called".