Speed improvement, smoother scrolling, and no freezing.
Multiple accounts management: If you have both a Gmail and Google Apps e-mail account, you can easily switch between them quickly. You will no longer have to use two different mobile apps to access personal and work e-mails.
Multiple mobile e-mail drafts: You can save multiple e-mail drafts in your mobile phone, so that you can pick and choose what you would like to send later.
Powerful shortcut keys: If you have a QWERTY phone, you can use shortcut keys. Hit 'z' to undo, 'k' to go to a newer conversation, and 'j' to go to an older conversation. See Menu/Help in the app for more shortcuts.
Basic offline support: Can't get a signal? Not a problem. You can compose and read your most recent e-mails even when there is no signal. Also, any outgoing messages will be saved in the outbox on your phone and sent automatically when you're back in coverage.
The offline support and multiple account management will be very useful. I've already downloaded this bad boy, and it seems a bit faster. Hop over to m.google.com/mail on your mobile browser if you want to download the new mobile Gmail.
Also, here's a Google-produced video explaining the new features: