I wouldn't call it truly open e-mail-killer solution if it's still a service provided by one company. If there was open protocol so that anywho could create their own wave service which worked with other wave services, I would approve it totally. Currently it's too dubious to have all your personal data in the control of a single enterprise.
...however I know that it would be harder to do that way, technically and in other aspects too.
As a web service, though, it looks promising, or more like incredible.