And yet even after writing Go for 5+ years I still have to Google for what actually implements Reader so I can read a file, etc.
Do you mean overloaded methods? You can "override" methods in Go in the sense that if you have an embedded struct you can provide your own implementation in your struct to use instead
If they did one thing exactly right on the language level, it's the [lack of] OOP: interfaces in their implementations, and no inheritance, overridden methods, covariance / contravariance games, etc.
You can of course write in Java in that style: only extend interfaces, never inherit classes. But many libraries, including the standard library, actively refuse to cooperate.