When I click a link far down in the article, read that for a bit, and then hit the back button I find it quite jarring that I end up back at the top of the original article rather that where I was. See if you can store scroll position on navigation.
They have managed to do it right and didn't cut corners. But it'll inspire thousands of "React developers" and provide them further validation to puke out abominations such as the New Reddit.