It's plugins done right with the gigantic caveats that it duplicates a large swath of low-level DOM APIs, requiring a large amount of effort to write two implementations of essentially the same things, and has no direct access to the DOM or external JavaScript, meaning it lacks support for some APIs (WebRTC) and cannot really be used as a drop-in replacement for JS or to take advantage of the existing HTML renderer (it's all or nothing).

That is, it's plugins done right if you insist on living in a separate process, despite the adequacy of both NaCl and JS engines' ability to keep code within the inner sandbox/VM, and what should be the adequacy of the sandbox of renderer processes even if someone manages to run native code in it.

Me, I'd prefer to just run emscripten on the Python interpreter, take advantage of a very small VM that nevertheless is apparently able to run code at half native speed, and start using a new language on top of all the nice existing stuff.