Microsoft officially announced RTM (released to manufacturing) for Windows Server 2012 R2. This means that the OEM partners get access to the gold bits. General Availability for Windows Server 2012 R2& System Center 2012 R2 is set for October the 18th. In the mean time, here are a some resources you can use to explore the features that have been added or updated in Windows Server 2012 R2.
MS TechNet Resources:
What's New in Windows Server 2012 R2
What's New in Windows Server 2012 R2 Essentials
What's New in System Center 2012 R2
You might only be interested in learning certain components within Windows Server 2012 R2. Here’s a list of topics broken down by core components to help you find the new features available:
What’s New in Hyper-V in Windows Server 2012 R2
What's New in Networking in Windows Server 2012 R2
What's New in Failover Clustering in Windows Server 2012 R2
What's New for SMB in Windows Server 2012 R2
New and changed functionality in File and Storage Services
To get a hands-on early look at some of the features, the Preview Release versions of Windows Server 2012 R2 and other products (System Center 2012 R2, Windows Intune, SQL Server 2014 and the Windows Azure Pack) are still available for download:
Microsoft Preview Evaluation Releases