I'm interested to see what the API side of WebAssembly looks like in browsers; hopefully this will make it easier to expose more full-featured sandboxed APIs to languages targeting the web, without having to tailor those APIs to JavaScript. For instance, API calls that actually accept integers and data structures rather than floating-point and objects.
For that matter, though in the short-term this will be polyfilled via JavaScript in browsers, it'll be fun to see the first JavaScript-to-WebAssembly compiler that allows you to use the latest ECMAScript features in every browser.
My hope is that WebAssembly will start motivating a set of lower-level APIs, with higher-level APIs being provided on top of those by WebAssembly "libraries". JavaScript tends to motivate much higher-level APIs, and you can't turn high-level APIs into low-level APIs, only the other way around.
For that matter, though in the short-term this will be polyfilled via JavaScript in browsers, it'll be fun to see the first JavaScript-to-WebAssembly compiler that allows you to use the latest ECMAScript features in every browser.