Unlike the other language translations, this is a new site design, but JP accounts work. And nobody has commented on anything yet, so go nuts.
As for English-speaking user features, it seems they’ve added Facebook login, and you can add Nico-style comments on Youtube videos. Which I’m pretty sure was a feature on nicovideo.jp until Youtube blocked them…
Also the videos don’t load half the time, just like the original.
A few classics: