In the end, I don’t believe that there are any drawbacks to virtualization as virtualization will save money and make server administration easier. However, just like any critical new piece of your infrastructure, server virtualization must be done right, from the beginning. You will need to spend time learning and training to learn about virtualization.
Likely, if you choose an enterprise virtualization product, you will need to spend money on virtualization software. It may also be necessary to purchase servers with more RAM (or upgrade existing unneeded servers) to be virtualization host servers (however you will be able to get rid of many servers with less RAM once they are virtualized).
If you do not already have one, you will likely need an iSCSI or Fibre Channel (FC) storage area network (SAN) to support some of the enterprise virtualization features. Also, keep in mind that as servers are more consolidated, if a single server goes down, you could loose as many as 50 virtual guest servers. With the servers being so consolidated, “all your eggs are in one basket” (or a few baskets), as they say. Because of this, you will need to choose servers that are as redundant and reliable as possible as your virtualization hosts.
Still, all of this investment in time, hardware, and software will be worth it, in the end, as you will realize so many benefits (see FAQ #2).