A clean, modern design
Visit Outlook.com and you'll immediately notice the sleek new design; the simple accents, eye-pleasing font and abundance of white space. This is important because we no longer just catch-up on our emails in a browser - nowadays we'll spend just as much time composing and checking mail on our phone (or tablet). So it's important for your Outlook experience to go with you and be available across all your devices.
And if you think the design looks familiar, you'd be right. Outlook.com appears to have utilised the same Metro design philosophy that is apparent throughout all of Microsoft's modern products (be that Windows Phone, Windows 8, the upcoming Office 13 or the Xbox LIVE dashboard).