> An example of how even a core package could do with improving: "image" doesn't support all forms of progressive and/or interlaced PNG

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