Well, in the way that Wave was centralized it would be nice if there was a package like that where you host it on your own servers. So secure data would stay in the company system. And in the event of an audit, the company would have an easier time hiding evidence of illegal activity, instead of going after the employee inbox. I think that is a feature Google can never provide!
Really I feel like the problems with email are that people use it for too many things at the same time. Sending messages, sending html formatted mailers, sending files, storing files, etc. If those were separated out into separate tools, even if they were in the same package, would be best. Though, this is for a more Intranet style setup.
Email is nice because it is ubiquitous. You can send emails to everyone from anywhere. The clients that read emails could be better designed to make emails less ugly as hell though. Because of this I can understand why so many people communicate through Facebook. It distills the messaging portion of email in to clean easy to view messages.
I dunno, there are still a lot of aspects to think about.