Go has a pretty decent standard library.
I ask this because I often find myself thinking I would want such-and-such a feature in a tool, library, etc, but unless it was implemented in response to an immediate need, when I actually add it I realize it ends up unused.
My guess is that better containers aren't in the library because of the lack of generics. I really hope the Go folks get around to adding generics soon; it's a beautiful language in most other respects. It would definitely be one of my first choices for a more systems-y project.