Great concept! Would be nice if it's an open standard. Instead of having to download and install your App, people can implement and choose from different implementations. This makes several things possible:
1. Different virtualization/container engine: Different platforms have different good stuff. I would use LXC/docker for Linux since it's super lightweight, and disposable. Even NaCl should be an option.
2. Standardizing would also help defining how resources should be allocated/re-used on local machines.
3. People who make web apps gets to use a single standard instead of tracking different technologies. (Think about Java Applet vs Flash vs Silverlight) Any site should be able to access it through a port on localhost. It's THE port, using THE protocol. And that's it; it should work on all hosts that have the service installed.
4. It brings a chance to have it accepted by major browsers makes as a core service, which makes it easier for wide adoption.