We've had such technology for decades. There have been desktop sharing apps for years, in Java applets etc.
So whilst it's cool that the browser wars are generating a ton of improvements, none of them so far are really things that will blow users minds. HTML5 is just catching up to where other platforms were 10 years ago.
Perhaps the focus shifted.
When I first saw Wolf3D as a kid I was amazed that the computer could do that. Same when I first used a GIS.
These days when people get "excited" about something like Facebook, it's not because it's a technical achievement. It's something more human. Which perhaps is why I don't get it.
Maybe browser technology is "good enough". Perhaps the point is to be able to get apps out to people as simply as possible. It's just a URL.
Due to the fact I don't feel a point coming on any time soon I'll end here.
Everyone knows that a sufficiently geeky person can make a computer do magical things.
However, apps (in the general sense) become much more exciting when they gain wide appeal, and the difficult discovery, install, setup, and maintenance experience of earlier approaches were an enormous drag on their wide success.
It's not the actual feature that will be new, but the simplicity is what will make it from a 'technical tool' in their eyes to some far more common.
It's not necessarily the DOM sharing, which is cool enough. I just think that the emergence of sockets in new browsers is going to bring in the "next level", whatever that ends up being.
Seriously incredible, especially considering the development timeframe.