An example of how even a core package could do with improving: "image" doesn't support all forms of progressive and/or interlaced PNG, any version of animated PNG, or WebP. The lack of progressive PNG means that you can't quite rely on it for user-uploaded images, and image/webp seems to be getting more traction and handling those appears to be growing in importance. Without these things, the other packages that rely on "image" to provide image processing can only be relied upon to work for JPG and GIF.
An example of an omission I found myself filling recently, a HTML sanitizer. I wanted something like the OWASP Java HTML sanitizer https://www.owasp.org/index.php/OWASP_Java_HTML_Sanitizer_Pr... , but could only find some very simplistic sanitizers packages (little to no ability to define the policy, simplistic URL checking, etc). I ended up writing my own package to help fill the gap https://github.com/microcosm-cc/bluemonday
You can tell I'm dealing with user generated/supplied content. Each domain still feels almost there (in having a toolbox of packages that do everything you need and are rock solid), but just shy of being there today.
Interlacing support for png will be in Go 1.4: https://codereview.appspot.com/102130044/
You can also follow: https://code.google.com/p/go/issues/detail?id=6293
For progressive/animated png-s please fill an issue: https://code.google.com/p/go/issues
EDIT: a pure Go-based LDAP library is what I meant. I know there's a couple on GitHub but they seem to be abandoned. Plus, there's another one providing bindings to the OpenLDAP libraries.