A year ago today, Microsoft introduced the preview of Outlook.com. User feedback has helped in delivering over 600 additions, changes and improvements, packaged in 34 feature releases in the last year. Over 100 of these 100 were to help improve performance, reliability and stability.
In terms of the size of the feature, here are the top twelve that have been delivered based on your feedback. Top 12 in the last 12 months
Source: Outlook Blog