I'd say my favorite thing is the effort on bringing generics.
I've wanted them for quite a while, since they will empower gophers to write more concise code.
Imagine a library containing common concurrency patterns, or maybe even some functional programming concepts like reduce or filter ... the dream!
I don't care that much about the error handling, because I think that once we have good generics we should be able to improve error handling thanks to other concepts like error monads etc.