Many users assume administrative time will be lessened, but in reality the virtual infrastructure itself has to be managed and may require a new centralized storage system, says Martijn Lohmeijer, managing consultant with TriNext, an IT outsourcing and consulting firm.

Frustratingly, many don't offer the same levels of support for applications running in VMs as they do for applications running on bare metal. Microsoft eased up on some last year, but analysts are still criticizing  and other vendors for restrictive policies related to support in virtualized environments. The calculation of software licensing fees can also be more complicated in a virtual data center.

"Not all server virtualization licenses are the same," DiDio says. "You have to really study the terms and conditions of your licensing contracts from the various vendors."

2. Consolidating onto fewer servers will be simple

The first goal of a server virtualization project is often consolidation. If you can run the same number of workloads on 10 servers that you were running on 100, why not consolidate as fast as possible? Unfortunately, many IT shops that plan to consolidate end up doing so much slower than they expect, says George Pradel, director of strategic alliances for virtualization management vendor Vizioncore. It's easy to say "every new workload has to go on a virtual machine," but moving old workloads from a physical box to a virtual one is not always a simple task, he says.