> I get that there's some novelty to this stuff, but we already know that asm.js can achieve performance within a factor of 2 of compiled C code.

My guess is that even if this is common knowledge (which I doubt), lots of people still don't know what that means. Demos like this showcase that.

As to JS vs. AS or Java Applets, well, among the "technical crowd" I'd wager that JS is more popular than those other two platforms. Simple as that, perhaps?