Fast hard drives
No index on hard drives
Plenty of memory
No Anti virus on the computer
Dedicated Virtual Machine (no other software running on that instance)
There should be no difference in resources allocated to the VM as you currently have in the physical machine.
You mention NAS, if that is the case then it will always be painful. They need to us SAN and a high-quality one at that so that the disk i/o is equal to or better than what you'd get from real internal drives.
Often IT departments will try to skimp on processor, memory and disk resources in order to cram many virtual machines onto one physical host. That's probably the biggest danger. FMS itself has no issues running on a VM.
Wim, interestingly - I asked for clarifiction re VM and NAS, as I had realised that NAS would have been a bad idea. No, turns out it was a SAN, and the slowdown was due to security software. But no details on i/o performance apart from that.
Additional info: It's been confirmed that they'll be using VMWare.