What are some considerations for effective planning and optimization of server virtualization?

Forgive me for providing only a brief answer. The big three area considerations for planning/optimization of server virtualization are:
CPU
MEMORY
STORAGE

Obviously the more CPUs or CPU cores you have in your virtual server, the more VMs (Virtual Machines) you can run simultaneously, and/or more CPU/cores you can dedicate to your VMs
Again, I think the installed memory is an obvious consideration, I would recommend 4gb per VM (server) at a minimum, and more as needed for simultaneous user memory consumption ( ie, a terminal server in a virtual machine, you'll want to spec the proper amount of memory based on projected concurrent usage )
Storage is the last leg of the tripod, and can have a dramatic effect on performance and usability ( either good or bad ). Beyond just capacity, you'll want to plan for disk i/o performance. For enterprise storage, faster spindles is better, so if you can use 15,000 rpm drives instead of 7200 rpm consumer drives, the data bandwidth will be better. Always when possible separate volumes per VM, and either separate channels or SAN ( fiber or iSCSI ) to prevent disk requests from stacking up on the drive channel. Optimize your volumes, so ideally you'd have one volume for your OS and applications, one volume for page file, and at least one volume for data. What you're trying to accomplish is also the same for a physical server, but virtual server configurations can make a poor configuration seem a disaster. ( you do not want a drive's read/write head thrashing back and forth across the platters looking for page file, then program, then page file, then data, then .... physical movement of the heads is exponentially slower than the read/write speed of the data and adds terrible delay)