Microsoft has posted a little bit of light reading, a document titled Changes in Functionality in Windows Server 2008 R2 that outlines exactly that, the differences between the R2 release and the original 2008 release. It only weighs in at 1.1MB and 211 pages. You can grab it from the Microsoft Download Center in Microsoft Word format or browse the document online through Technet.
Topics such as new features in Active Directory, DNS (including DNSSEC), Group Policy, iSCSI, IIS, clustering, Windows Deployment Server and many other elements are all covered in great detail. If you’re looking at a server wondering if you’d see any significant benefit to upgrading, other then getting the latest guts of Windows, this is a helpful guide.
Windows Server 2008 was released in February 2008, followed by R2 which was released in October 2009. R2 is Microsoft’s first 64-bit only operating system release and is based on many of the same core programming as Windows 7.