It's no surprise that an HTTP library doesn't have issues with the lack of generics -- it shuffles text around. Reusable data structures are the obvious case for genericity, and I don't see many in the docs. Once you have a higher-order language (whether via objects or first-class functions), some control structures become data you might want to use generically. Looking over Java's stuff also shows some interfaces one might want to parameterize over a type (e.g., Comparable -- what can you compare this to?, Future -- what type will we get from it?). Haskell also offers a lot of examples of really exercising polymorphism (like lenses, but those maybe aren't appropriate for Go).