One of the abilities that were really missing for me when I was thinking about Windows Phone 7 Applications to develop was the ability to browse through the user’s contacts and appointments databases. This ability will now ship with the Mango update. Developers will be able to browse through and conduct searches over these databases. This is, in my opinion, a very important update that can allow many missing apps which exist in other platforms to be added to the Windows Phone Marketplace.
One thing which will not be possible, though, is accessing 3rd party social networks data (such as linking a Facebook profile to a user).