Designed for mobile (currently iPhone + gmail only) and built loosely around the inbox zero concept, Mailbox have:
"redesigned the inbox to make email light, fast, and mobile-friendly. Quickly swipe messages to your archive or trash. Scan an entire conversation at once with chat-like organization. Snooze emails until later with the tap of a button."
"The big a-ha was discovering that the primary-use case of email on the phone was really triage," he explains. "It’s taking those couple of minutes that you have to go through your mail and trying to look at what’s new that you need to worry about--and what you don’t need to worry about."
And before you ask, no I don't have it yet.