The app, which is compatible with OS X Mavericks and later (sorry, Mountain Lion users), is double-paned with a rail on the left side. The latter lists your folders, drafts and archives, while the former handles your inbox (or whatever folder you open) and the body of individual messages. It tries to maintain a minimal look, much like what you'd see on the mobile version, and it even comes with the same swipe gestures (both regular and long swipes are supported). There are plenty of hotkeys, and Mailbox makes it easy to remember which shortcut takes you to which feature: As soon as you hit the command key, each icon on the screen transforms into a letter or two that serves as a hint at what you're supposed to press.
Mailbox has also integrated a feature that lets you hit snooze on an email on your phone and it'll reappear when you get back to your computer (and vice versa). This makes it easy to push off a message until you have the time and ability -- and faster typing with the keyboard -- to take care of it efficiently.
A desktop email client also needs support for writing drafts, and Mailbox made sure to include it not only in the Mac beta, but in the mobile apps as well (on iOS right away, with Android coming "very soon"); company reps said that they wanted to keep the experience as consistent across platforms as possible. The only catch is that you'll need to be signed into your Dropbox account to save your drafts, since it'll use up your cloud storage space.
Mailbox reps didn't want to nail down a specific timeline for when the final version will be ready to ship, though they felt confident that it won't be too long. But since the beta is all about getting feedback from early users and fine-tuning the app, the company's more focused on making sure it's done correctly, rather than quickly.