*** Update: Service Pack 1 re-released fixes Search issue.
Compatibility with new stuff:
- Support for Windows Server 2012 R2
- Fixes for users with Windows 8.1 and Internet Explorer 11
- Service Pack 1 rolls up previous cumulative updates and hot fixes. No need to install the March 2013 CU for SharePoint before installing later updates.
- Support for OData JSON light – allows developers to select the level of metadata returned by requests.
- Ability to turn off the SharePoint News Feed and replace it with a Yammer feed.
- SkyDrive Pro has been renamed to One Drive for Business
SharePoint Service Pack 1 can be downloaded from TechNet.
Related to this is Service Pack 1 for Office 2013 (including Office RT for Surface users) which was released at the same time.