One of the most heralded features in Windows Server 2012 (previously known as Windows Server 8) is support in Hyper-V for virtual machine replicas. If you’re familiar with VMWare it’s similar to Site Recovery Manager, for Xen you’re probably looking at Remus, and Kemari for KVM (bear in mind that none of these products are necessarily exactly the same, but close enough).
Virtual Machine replication across sites is very attractive. You get a lot of flexibility with minimal effort. You don’t need to learn about making individual services you run on a given virtual machine highly available. However users don’t care about a server being highly available across multiple sites. They care about their email, their documents, the company database(s), etc. They care about the services.
With a single machine it’s entirely possible for it to get pwned, for an accidental misconfiguration, or any other number of things that causes a service to become unavailable. As has always been multiple servers providing a service helps negate some of these issues.
None of this should be new information, but with the allure of “new” toys (in Microsoft’s “free” virtualisation tech) it should not be forgotten. Having spoken to several clients, co-workers and peers in the Windows world I fear it’s a lesson that some admins may be forgetting.
TL;DR As with any technology, use virtual machine replication wisely and, most importantly, use it appropriately. Don’t forget about service/application level replication.