Looks really cool, but I wouldn't go so far as to classify NodeJS as a "toy" solution. Using your definition, Sinatra is a "toy" solution, as is default PHP. None of those have "deployment processes, multi version processes, system setup, server tuning, database scaling, backup processes, security policies, monitoring, profiling, admin console etc, etc." baked in either.
Ok, it was a 'play' of words. I wanted to point out that after you put 90% of the effort to create an application you need to put 90% effort to actually deploy it in production. Google App Engine helps here.
But the fact that AppengineJS (and Rhino/RingoJS that powers it) runs on GAE is a distinctive advantage IMO.
Maybe there's a grain of truth to it, but it doesn't particularly convince me fairly well. First of all, you're presuming that it's a good thing to share code on the client and server side. It turns out that there is such thing as reusing code too much. In particular, I don't like the idea that now not only do I have to worry about browser-specific hacks in my client-side code, but I have to also worry about browser-specific hacks in my server-side code as well.