-) A proper package manager. (no, go get is not enough. They have realized this and have a team coming up with a draft for an official one)
-) Well written, well maintained libraries that do proper releases. There are very few of those out there. Might get better with an offical package manager and ideally package repository a la npm/crates.io
-) Generics. This will help the previous point also. Often it's hard to write proper reusable libraries due to the lack of generics. And no, they didn't keep generics out because they are to complicated for the language users. They just (quote) "didn't find a good way to implement them", and big language changes like that are out of scope before 2.0.
-) Macros probably won't ever come, but if not, please at least give us a language construct that reduces error handling boilerplate. (Rust has some nice macros for that).