While doing my morning news scan, I came across Top 10 Server Virtualization Myths -- Debunked posted by Kenneth Hess over at ServerWatch. He brings up some important issues concerning cost, performance, security, reliability and even licensing issues and points out how they can be tackled. I believe we'd both agree that those who have built their environment based upon a sound plan or architecture are more likely to be successful than those who allowed their environment to grow organically without a plan.
The post really doesn't go into issues related to other forms of virtualization, such as access virtualization (an important component of VDI or desktop virtualization) and application virtualization. It does, on the other hand, touch on management of virtual environments and storage virtualization.
It might be worth your time to read this short piece.