Interesting, because a common criticism of Go is that it's "stuck in the 70s". [1] It's "modern" only because of its age, not because of its features.
But that's not to say it doesn't include good stuff.
I'd say support for lightweight threading, CSP (channels), structural typing, fully embracing first-class functions (crippled python lambda, anyone?), and a build-system integrated into the language rather than left to 3rd-party-tools are all distinctly post-90s features, for a mainstream language.
Also, gofmt is a huge boon. The Go community focuses on ecosystem and ergonomic human factors of development feels distinctly post-90s-lanauge to me.
Also, the 90s saw an intense, misguided focus on "object-oriented-programming", which Go completely, mercifully ditches. In that sense, it's post-90s. :)
(Well, OK, maybe OO-style programming is good for UIs -- thinking of stuff like iOS/Cocoa apis and Unity.)