Exchange 2013 builds on the existing High Availability and Site Resilience features from Exchange 2010. A huge amount of work has been done in many areas to provide better availability and failure isolation.
The use of DAGs and database copies along-with features like Single Item Restore, lagged copies, retention policies continue to provide HA and SR. The high availability platform, the Exchange Information Store and the Extensible Storage Engine (ESE), have all been enhanced to provide greater availability, easier management, and to reduce costs. These enhancements include:
There are a few more that you can read about on http://technet.microsoft.com/en-us/library/dd638137.aspx
Useful information. thanks