What doesn't work yetSCVMM 2012 R2 does not support the management of Storage Spaces for Scale-Out File Servers on Windows Server 2012, leaving your options at Server Manager or PowerShell. The Physical to Virtual (P2V) functionality is being retired with SCVMM 2012 R2, so you'll have to find some other way to convert existing physical machines into virtual ones if that's a requirement. You can't manage the new storage tiering or write-back cache features of Windows Server 2012 R2 directly with VMM. You can still do that with PowerShell, and in theory, you could automate scripts through VMM to accomplish the task if necessary.
When adding a new host, System Center Virtual Machine Manager 2012 R2 will check for hardware support of features such as Multipath I/O and let you know if they're not enabled.
You cannot live migrate a VM from a Windows Server 2012 R2 host to any earlier version of Hyper-V. You can go the other direction, from a Windows Server 2012 host to a Windows Server 2012 R2 host. For live migration to work, you must have both host systems configured to the same logical network. This means you must have used the same naming convention when you created your virtual network on that host. That's one more good reason to use SCVMM to manage everything -- including your networks. SCVMM 2012 R2 does not support creating VMs that use differencing disks. You can accomplish the task with either Hyper-V Manager or PowerShell.
The list of features in Windows Server 2012 Hyper-V R2 fills in many missing pieces from the initial release. It's still just a preview, and bugs are to be expected. That said, I can attest it's a remarkably stable release, based on a number of weeks of heavy testing. In times past, you'd expect to have regular blue-screen crashes with an initial "preview," but that has not been my experience so far. The new features seem to work as advertised along with the improved ones. Many companies frequently wait for the first significant update of a major OS release before they begin any serious testing. Windows Server 2012 R2 and Hyper-V R2 seem to fit "significant" quite well.
SCVMM 2012 R2 gains incremental supporting updates, but nothing earth shattering. Nevertheless, for larger environments or for shops planning to leverage Hyper-V's new networking capabilities, it will need to come along for the ride.