New features introduced in Exchange Server 2010.

Client Access Server Role

1. Client Access Server (CAS): High availability for the Client Access Server role is provided by using Client Access Server (CAS) arrays. A CAS array can contain multiple Client Access Servers in an Active Directory site and provide a single name endpoint for client connections. CAS arrays cannot span multiple Active Directory sites. 
2. RPC Client Access: With the introduction of the RPC Client Access service, all Outlook clients access their mailbox database through the Client Access Server role. This abstraction layer allows for improved load balancing and redundancy and minimal client impact in the event of a database level *-over ("switchover" or "failover") event. 
3. Outlook Web App includes improvements: Including, for example, the ability for users to track their sent messages and printable calendar views and the "Premium" experience is now available across multiple browsers (including Safari and Firefox).

Hub Transport Server Role

1. Shadow Redundancy: Exchange Server 2010 introduces a transport concept called "Shadow Redundancy" that protects e-mail messages while they are in transit. If a Hub Transport server or an Edge Transport server fails after it has received a message for processing, but before it was able to deliver it to the next "hop" server, the server sending the message to the transport server is now able to detect the failure and redeliver the message to a different Hub Transport or Edge Transport server for processing.

Mailbox Server Role

1. DAG (Database Availability Groups): SCC, CCR, LCR and site resiliency functionality SCR have been replaced by DAG. It provides database-level high availability (as opposed to server level) and supports a number of copies of each database (number based on Exchange Edition) and flexible configuration (databases copies may be added/ removed at will without requiring major server reconfiguration). 
2. Personal Archive: Exchange Server 2010 extends the large mailbox support introduced in Exchange Server 2007, and also introduces a Personal Archive feature to allow messages to be retained longer without the need for a third-party archival system. The Personal Archive is implemented as a secondary mailbox for archive-enabled users, and in Exchange Server 2010 Service Pack 1, the Personal Archive may be located on a different database than the primary mailbox, which may reside on a different disk if desired. Backup can be performed via multiple solutions like Handy Backup or Acronis. 
3. Recoverable Items: The compliance and legal search features have been enhanced. What was formerly known as the "Dumpster" in previous versions of Exchange (a special storage area for messages that have been deleted from the Deleted Items folder or "permanently deleted" from a regular folder, such as the Inbox) has been evolved into the Recoverable Items folder in Exchange Server 2010. If configured appropriately, the Recoverable Items folder allows for a "tamper proof" storage area (users cannot circumvent the Recoverable Items folder to bypass legal discovery), which also provides a revision history of any modified items. 
4. Mailbox Server Role may be combined with the Client Access Server: In Exchange Server 2007, a clustered mailbox server could not be combined with any other roles. In Exchange Server 2010, the Mailbox Server Role may be combined with the Client Access Server and/or Hub Transport roles, regardless of whether or not the mailbox server participates in a Database Availability Group. However, since Database Availability Groups use Windows Failover Clustering, and Microsoft does not support the combination of Windows Failover Clustering and Windows Network Load Balancing on the same server, a multi-role deployment will require the use of a 3rd party load balancer to provide load balancing and fault tolerance for the Client Access Server role.

Other New Features

1. Cost savings in required hardware: Exchange Server 2010 provides cost savings in required hardware. Storage performance requirements (measured in IOPS: Input/output operations per second) have been reduced by approximately 70% over Exchange Server 2007, and by approximately 90% over Exchange Server 2003. According to a case study, Microsoft IT was able to reduce hardware costs by 75% during the migration from Exchange Server 2007 to Exchange Server 2010. 
2. Administration delegation (RBAC): Can now be performed at a granular level due to Exchange Server 2010's implementation of Role-Based Access Control (RBAC). Users and administrators can be given extremely fine-grained abilities for functions provided both within the Exchange Management Console or Exchange Management Shell and in Outlook Web App. For example, a compliance officer may be given the ability to perform cross mailbox discovery searches within Outlook Web App; a help desk technician may be granted the ability to set an Out Of Office message for other employees within the company or a branch administrator in a remote office may be granted the permission to perform specific Exchange Management Shell commands that pertain only to the Exchange server in their branch office. 
3. Distribution groups can now be "moderated": Meaning that distribution groups can now be configured to allow users to join at will or only with a group moderator's permission, and individual messages sent to distribution groups can now be approved or denied by a moderator.