My problem with Inbox is that it doesn't let you easily delete emails. It's 2 or 3 actions to delete, which is unreasonable with high mail volume. The Google party line is that you should never have to delete, but I don't care to have old quora digests in my archive when I'm searching for actual relevant mail.

In addition, the snooze feature for Mailbox is extremely helpful.

Quite frankly - speed. Mailbox on the mobile is just blazing fast.