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> From what I can tell there are no instructions in the app that tell you about this feature

It's fine for secondary controls (other ways to zoom). I'd say Google learned the lesson with the now discontinued Menu button on Android. Application authors would sometimes hide critical functionality behind a Menu button press, and sometimes the Menu button did nothing. This lack of discoveribility and inconsistency is bad for User Interaction

> Another example, a great feature in Google Maps on mobile is the ability to zoom with one hand. You double tap and drag (tap, tap-and-hold, drag), but I only figured this out because some blog talked about it.

This is a native Android gesture, by the way, and works in most apps and web-pages. It used to work on HN, but stopped when they changed the markup to be more mobile-friendly.

>But that's kinda the thing; how do you end up finding out about them?

Someone shows you, you see someone do it, you accidentally press and activate it, you watch the introductory "wizard", you see it in an iPhone ad -- there are literally tons of ways.

Including the one everybody hates, RTFM...