If you have seen Office 2013, you know the Metro-style look has crept into the client, and OWA is no different. But new to 2013 is the ability for OWA to continue to function offline, which it does via HTML5, making it the most capable mail client for the Windows RT platform at this time. (It works on Windows 8, too, but regular old Outlook would be the clear choice there instead of OWA.) The offline feature works in Internet Explorer version 9 and up, in Google's Chrome browser version 18 or later and in Safari version 5.0.6 or later.
In addition, OWA now supports "apps," which are basically tiny pieces of code that try to sense what you are doing within OWA and offer additional, context-sensitive functions. For example, if it sees you have received a message with driving directions, it offers to open up a map; or if it detects a task or an action from a message, it will add it to a Suggested Tasks list.
Developers will be able to create their own apps to bring even more functionality to OWA. These apps will live inside Exchange's "mailbox store" for each user and will be independent of the specific version of Exchange Server, so future upgrades and updates should not break this functionality.
The number of server types and roles has been reduced. Previously there were a few different types of Exchange servers that could make up a deployment, including mailbox servers (where messages lived), hub transport servers (where messages and items were routed between other Exchange servers in an organization, especially when there were multiple physical locations involved), edge servers (which caught messages coming in from other systems, including Internet mail) and unified messaging servers (which hosted voice mails, IP and Voice over IP calling plans, and other voice-related tasks).
In Exchange 2013, there are now simply mailbox servers and client access servers (CASes), and there is no longer a hub transport role or a unified messaging role as there was in Exchange Server 2010. Fewer pieces equal less complexity, a boon for administrators who had to manage multiple server types in a variety of deployments across the world. It also makes patching and regular maintenance easier, with fewer possible points of failure.