It's open source in the sense that the source is available. It's not open-source in the sense of being a community project. As for Javascript vs Java, Javascript has come a long way. Recent JS engines are pretty fast. Not to say that it can't be tricky to maintain a large base of JS code, but that's a different debate.
I for one am looking forward to it, though the lack of apps compared to the other two dominant ecosystems will be a problem.