Hyper-V Remote Management
It turns out that you cannot manage a Windows 2008 R2 Hyper-V installation from a Windows 8 desktop (even with the Remote Server Administration Tools for Windows 8 installed) or from Windows 2012 Server.
You can, however, manage a Windows 2012 Hyper-V installation from Windows Server 2008 R2. While I’ve not tried it, I suspect that this means that you can do the same from Windows 7 RSAT.
This means that people with an estate containing mixed servers won’t be able to upgrade their management desktops to Windows 8.
With a bit of luck, Microsoft might notice this oversight and fix it soon.