One of the problems associated with virtualisation is "virtual sprawl" — the proliferation of virtual machines which can totally negate the idea of server consolidation if not carefully controlled. Management becomes critical, and in particular, patch management. When virtual machines are not running, they don't receive Windows Update patches, meaning they can quickly become out of date.
Microsoft has released the Offline Virtual Machine Servicing Tool to address this: it can automatically start offline VMs, patch them using Windows Server Update Services (WSUS) or System Center Configuration Manager, and then shut them down again.