WebAssembly expands on the ability to do client-side stuff and makes it more accessible, so my fear is that it will make it more common.
That's pretty much the entire difference. It's a difference of degree more than of kind. As I said, Javascript is making much of the web inaccessible to me, and WebAssembly, if it becomes popular, will only accelerate this loss.
WebAssembly adds new functionality to the browser runtime environment that isn't available to regular JavaScript. It was originally 1:1 with asm.js/JS but that stopped being true relatively quick.
WebAssembly allows allocation of memory and exposes a huge threat surface. We can't even protect the current APIs and these are being exploited by sovereign nations to oppress people like the Uyghurs. How about we focus on making the sandboxes stronger instead of building out things like WebGL and WebAssembly.
That's pretty much the entire difference. It's a difference of degree more than of kind. As I said, Javascript is making much of the web inaccessible to me, and WebAssembly, if it becomes popular, will only accelerate this loss.