What’s new on the fly:

 

  • Server Roles: Microsoft Exchange Server 2013 Preview consists of
    the following two server roles Client Access server role and Mailbox server
    role Exchange2013 Server Roles

 

  • Exchange Administration Center: No Exchange management console
    ,Exchange 2013 Preview provides a single unified management console that allows
    for ease of use and is optimized for management of on-premises, online, or
    hybrid deployments. The Exchange Administration Center (EAC) in Exchange 2013
    Preview replaces the Exchange 2010 Exchange Management Console (EMC) and the
    Exchange Control Panel (ECP). ExchangeAdministration Center

 

 

  • Exchange 2013 architecture  RPC is no longer a supported direct access
    protocol.     This means that all Outlook connectivity must take place using
    RPC over HTTPS (also known as Outlook Anywhere).

 

  • New public folder design: Public folders in Exchange 2013 Preview
    now take advantage of the existing high availability and storage technologies
    of the mailbox store. The public folder architecture uses specially designed
    mailboxes to store both the hierarchy and the public folder content. This new
    design also means that there is no longer a public folder database. Public
    folder replication now uses the continuous replication model. Public
    Folders    .

 

  • 2013 Preview has *Offline* access in Outlook Web App (OWA).

 

  • The transport pipeline in Exchange 2013 Preview is now made up of
    several different services:     the Front End Transport service on Client
    Access servers, the Hub Transport service on Mailbox servers, and the Mailbox
    Transport service on Mailbox servers. For more information, see Mail
    Flow    .

 

  • Site mailboxes improve collaboration and user productivity by allowing access to both documents in a SharePoint site and email messages in an Exchange mailbox, using the same client interface. A site mailbox is functionally comprised of SharePoint site membership (owners and members), shared storage through an Exchange mailbox for email messages and a SharePoint site for documents, and a management interface that addresses provisioning and lifecycle needs.see Site Mailboxes

 

The whole list is here http://technet.microsoft.com/en-us/library/jj150540(EXCHG.150).aspx#BKMK_Share