What about I/O
http://virtualgeek.com posted an interesting issue.
A sneak peek at vSphere 4 I/O goodness (and world records)
So, about a year ago (May 2008), the VMware Performance engineering team and EMC collaborated on an effort to see just how much IO you could push through an ESX 3.5 host. The results of that effort were posted in detail here, and it started a lot of dialog about what applications were candidates for virtualization.
Some of you might have seen this IOmeter "speedometer" that represented the maximum IOps (100,000) we were able to push through a single ESX 3.5 host.
Click on the hyperlink for the rest of it