The one thing that rubbed me the wrong way is the "let" keyword. Why can't we just declare variables without it, like C does? I suppose because they want to rely on JavaScript's own scoping rules?
What would be cooler would be if they could just totally do away with making actual JavaScript variable declarations or function calls, save for the set of typed arrays, pushing and popping variables on and off the stack for function calls, and then coming up with some horrible hack for implementing jmps in a language with no goto. I'm sure this wouldn't really make things more efficient, but it'd be interesting to read.
I guess though, there's always the source of emscripten for that delight.