This week I met a Microsoft NZ person who gave me a Windows Phone Mango demo... Lots of new features, but a picture (movie) is worth a thousand words:
Seriously, the integrated inbox (which allows you to select which inboxes are part of the integrated view, and allows you multiple integrated views) and the contacts groups are beyond anything you've seen in any other smartphone platform. Contacts allow you to see all communications with contacts - email, IM, social networks, etc.
And on communications, you can start a conversation via IM, and if the contact goes offline it will automatically send the next message via SMS, or via Facebook IM, or whatever is available. And you see all in a single thread.