Along with Office 2013 SP1, the Office servers were also updated to SP1 in February 2014, including Exchange, SharePoint, and Lync.
Most of the Exchange updates are on the server side, but some specifically apply to end-users: The rich text editor in SharePoint is now used with OWA, allowing users to create tables, preview linked content, and have more options for paste. It's actually not too bad to use. S/MIME support in OWA is back. The other big addition on the end-user side in involves mail apps: Developers can now build apps that will help users when they are composing email.
On the server side, Windows 2012 R2 is supported, the Edge Transport role is back, cmdlet logging is available in the Exchange Admin Center, and improvements were made to DLP.
Plus, SP1 introduces a new communication method between Outlook and Exchange 2013: MAPI over HTTP(MAPI/HTTP). This should improve the user experience when switching networks or resuming after hibernation. It's disabled by default but can replace RPC/HTTP (Outlook Anywhere) for Outlook 2013 SP1 clients. Outlook 2013 RTM and older clients will continue to use RPC/HTTP. It should be a lot easier to implement and creates more reliable connections.
Technical details for MAPI over HTTP are available at MAPI over HTTP
Download Microsoft Exchange Server 2013 Service Pack 1 (SP1)