Mostly, though, I'm struck by the way this suddenly makes phishing scams (and money-based spam in general) more frictionless -- it now becomes easier to impulsively respond to an email by sending money. I give it a week, tops, before we start seeing spams telling woeful stories of how you can help a poor starving child by just sending $3 now -- it's quick and easy! And I bet it opens up whole new horizons for money laundering. I hope Google has enough staff laid on for all the regulatory fun they're likely to hit.
(Man, I really was working at Memento too long -- I've gotten remarkably cynical about anything involving money...)