Defaults are important because so few people change them or even notice them. Hiding the URL bar by default will make some new Chrome users not even realize it is there, and yet more will eventually neglect to use it properly. The web then becomes like a HyperCard stack, with Google as the top card (by default). I'm sure this is exactly how Google wants the web to be, but it's not going to help create knowledgable web users.
I'm sure you've heard of users that google "www.nytimes.com" to go to the New York Times, because they already don't know how to use the URL bar or what a URL is and equate Google with getting anywhere on the web. Think about how this change is going to influence people of that technical caliber.
I did just think of one problem though. Phishing will be even easier so they'll really have to step up their detection and warnings for those.
Of course, I also think this is a pie in the sky mockup that won't ever make it to prime time due to numerous usability problems, but who knows?
For the most part, Chromium development happens in the open. There are many interesting ideas, but more newsworthy would be actual (recent) developments.