I think the statistical importance is by far the most interesting feature. Our inboxes are really priority queues, but we are currently spending a great deal of time and energy doing the prioritization ourselves. The idea of seeing my inbox sorted by importance is game changing.

Hilary, what are the aspects of the email that influence importance? Is it mostly the text content of the email, or are there things like sender, number of emails in the chain, number of recipients, etc?