I think Go has made a lot of people re-think their assumptions about what makes a language a good language.
I have yet to figure out precisely what it is that makes me more productive in Go (which I have relatively little experience with) than Java (which I've programmed since the first beta).
At least one thing I can say right away is that they have taken care of a lot of small hygiene factors. Things like the rapid compiler, the "go" command that has a lot of nice things built in, the way they did away with formatting discussions (although I can't articulate how it differs from other languages that try to do the same....probably the tooling).
The Go community also prefers libraries over frameworks. Coming from Java, and its humongous, cancerous framworks, that's a BIG selling point. I notice that when someone posts a link to a "Go framework" I get slightly ticked off and I feel resistance even before I've clicked the link.
As for the language: I don't know yet. It just works for me. :)
I have yet to figure out precisely what it is that makes me more productive in Go (which I have relatively little experience with) than Java (which I've programmed since the first beta).