I find many ideas in this proposal intriguing and good.
Still, I'm very concerned about the recent trend to funnel everything through HTTP, Json/XML and the browser. I think t's the wrong direction.
There are quite successful models of signed, content-addressable storage around (peer-to-peer networks, Git). There are many other protocols on top of TCP/IP worth considering. And there is a working key distribution model around (pgp, web of trust). Wy re-invent everything?
Peersonally, I am not thrilled by the prospect of having the fat, bloated Web browser as my "second-layer" operating system.
 Of course, Google *likes* this trend :-)