Offline support in OWA : Emails and actions are automatically synced the next time connectivity is restored.
Client connectivity : CAS role is the point of connectivity for all clients in Exchange 2013.
Public folders : In 2013, public folders are now part of mailbox databases and high availability is achieved using DAG.
Site Mailboxes : Brings Exchange emails and SharePoint documents together.
Outlook Web App : Offers three different UI layouts optimized for desktop, tablet, and mobile phone browsers.
Ability to customize : Outlook and OWA by integrating apps from the Office marketplace. (Yes, this is a reference to the Agaves add-ins that Microsoft and partners will be making available via the new Office Store.) The new "Napa" tools and/or HTML5 are Microsoft's preferred ways for developers to build these.
Exchange Administrative Center (EAC) : Replacement of the Exchange Management Console by a Web-based Exchange Administrative Center (EAC).
Support for up to 8TB disks : And multiple databases per disk via Data Availability Group (DAG) management.
Built in basic anti-malware protection : Ability for administrators to configure and manage settings from inside EAC. (Note: this feature can be turned off, replaced or "paired with premium services such as Exchange Online Protection for layered protection.").
New Data Loss Prevention (DLP) : Capabilities for identifying and protecting "sensitive data." DLP policies are based on regulatory standards, including PII and PCI. Also: new policy tips in Outlook 2013 can be set to inform users about potential policy violations.
In-Place eDiscovery : Can be run across Exchange, SharePoint, and Lync (now Skype for Business) from a single interface.
Combine Roles : A reduction in the number of available roles to two: a Client Access Server and a Mailbox Server role.
FAST Search : Now integrated into Exchange 2013 managed store to provide a more consistent (across Microsoft servers) indexing and searching experience.
Inclusion of a "Managed Store" : The name of the rewritten information store processes, which are now written in C#.
Replication : Public folders are now stored in mailbox databases and can take advantage of Database Availability Groups for replication and high availability.
Data loss prevention : Capabilities that can be integrated into Transport Rules.