Outlook.com, which is no longer in preview phase, has seen over 60 million users to date and is Microsoft's answer to a more modern private email solution to Google's Gmail and Yahoo Mail. Microsoft has put up a comparison of Outlook, Gmail, and Yahoo Mail, and not-surprisingly Outlook.com comes out on top.
For example, one of the features that Outlook.com has that Gmail or Yahoo do not have is the lack of targeted ads based on email contents. Other features that are exclusive to Outlook.com are the ability to share, view, and edit Office documents via Office Web Apps, as well as the ability to send photo slide shows from your inbox. Outlook.com can also delete email based on time, such as "delete after 3 days." There is also the ability to connect your inbox to Facebook, Twitter, and LinkedIn, which is not possible with Gmail or Yahoo Mail. What Microsoft does not mention is that there are advertisements and if you want to get rid of them, you can "upgrade" to an ad-free Outlook.com for $19.95 USD a year. Hit the source link to see the chart, created by Microsoft.
On a side note, for those interested, Microsoft has also put together details on how one can migrate from Gmail over to Outlook.com, which features a step by step guide on how to make the switch.