Not to be too wet of a noodle (and I'd certainly like Outlook to go away), but replacing Outlook at places where it is currently entrenched will require an absolutely Herculean effort. Not so much because Outlook the mail client or mail server is so great, but because of everything it ties into: calendaring, sharepoint, lync, custom LOB apps programmed to the Outlook APIs, etc.

When trying to talk people away from Outlook that entire secondary ecosystem has always been a much bigger roadblock than the lack of a native client or even the idea of storing corporate data in "the cloud" in my experience.