Gonna be honest, my productivity app once upon a time was unsaved sublime text documents
Must be a me thing, then.
I have seen colleagues using an almost append only txt file with notepad.exe. It worked for them I guess, but there were some features I could not live without on Notepad++
What have you moved on to?
IMO the platform is unmatched at rapid on-demand WYSIWYG visualization.
Not so great for a productivity app, though. Too easy to lose important information when it's on the same sheet of paper as a drawing of a graph algorithm that turned out to be wrong, and trying to remember whether x cross y positive implies x right or left of y.
That would make it easier to not lose information, but I don't think it makes it any easier of a productivity app.