To answer your question a bit more specifically, one thing that's starting to work as a nice discovery loop in the app is being able to tap on a commenter's profile and see what other Substacks that person is subscribed to. Reader profiles existed before the app, but since comments don't render in emails (but do in the app), this discovery loop was pretty constrained.

We have some other exciting ideas in the works for helping readers discover more writers through the lens of the writers they already trust, that the app will provide a nice canvas for.