Go has nothing unique (and maybe that makes it special). I mean that sincerely, everything it has, has been done dozens of times. Interfaces in Go are implicit (which is important, IMHO) and really, tremendously simple. These two features make them exceptional easy to use, and ACTUALLY used.

I have used interfaces (or equivalent concepts) in dozens of languages and they always felt like far more of a chore, explicitly using X interface or Y interface, ugly complex declarative specs, etc. Go just makes it painless.

You can make an io.Reader interface in any language. Interfaces don't really shine until you start composing them.

Say you have a type that implements the io.Reader, io.Writer, io.Closer and io.Seeker interfaces, eg. an os.File. This type would also automatically implement io.ReadCloser, io.WriteCloser, io.ReadWriter and io.ReadWriteCloser, io.ReadSeeker, io.ReadWriteSeeker, io.ReadWriteSeekerCloser, io.WriteSeeker, io.SeekCloser etc.