The sneaky Windows R2 feature upgrades. Their minor version change belies the extent of the upgrade and the effect it can have on your environment. Many shops are happy to stick with a version of Windows Server that is older and serves its purpose, but if you're running a virtualized environment with Windows Hyper-V, you need to upgrade frequently to gain critical new features.
With the beta release of the Hyper-V role in Windows Server 2008 (and the subsequent official release in 2008 R2), Microsoft entered the virtualization game behind its competitors. It was a usable product but it lacked many advanced and/or convenient features required to run VMs at scale. It was an important first step however and they began to iterate on the platform pretty quickly with each new version bringing large improvements over the last. When Windows Server 2012 arrived, it brought Hyper-V into a mature state with most functions on par with VMware and Citrix. With Windows Server 2012 R2 and Hyper-V 2012 R2, the product has become one of the best available.
Among the list of new features in 2012 R2, the primary reasons we made the move to the new version in our data center were the improved Live Migration speed, better Linux support, and the ability to re-size virtual hard drives with the VMs still running, and Storage QoS. To make things easy, Microsoft has included the ability to Live Migrate VMs from Windows 2012 -> 2012 R2 with no down time, nice!
Except that they rarely mention the fact that once a VM is on R2, it can't be migrated back to an older version. In addition, you can't manage a 2012 R2 installation from a 2012 server, you just get failed tasks like crazy with no mention why. This is less of a problem when the new machine is a full install of Windows Server 2012 R2 since it can manage itself, but if you install Windows or Hyper-V Core 2012 R2, you can find yourself stuck without any management tools. If you find yourself in this predicament, I suggest spinning up a new VM on one of your 2012 hosts but have the VM run 2012 R2 which you can download for free from TechNet for a 180 day evaluation. Then you can use that VM to configure your new host if necessary.
This may sound like obvious advise, but it was not at all clear to me that this was the case when we deployed a new host on 2012 R2. The generic error messages and task failures left no hint of the root problem and the confusion was enhanced by the "cross-version live migration support" feature which sounds like you can move between versions. In short, the upgrade is well worth it for the new enhancements you get for virtual machines, just be prepared that you'll need to upgrade all of your host machines.