Final version of Outlook.com includes the ability to send large files, auto update address books and combine social media accounts and professional accounts like Twitter, LinkedIn and Facebook. A big feature that is received well by users switching to Outlook.com is that fewer ads are seen (as less as 60% then before).
It was launched in July 2012 and has seen rapid adoption by users, one factor can be because Hotmail accounts are also moved to Outlook and besides that, Windows 8 offers an integration with Outlook account. The main idea here seems to completely tie up MSN.com, computing on Windows and Hotmail services into one unified user experience.
Now that Outlook.com is no longer in preview, it will be interesting to see how it can cope with large user base and provide reliability (like Gmail).