if you read the blurb you can virtualise everything. the reality is if you have two applications that want the same resource all of the time you are introducing a contention and you will see a problem. It will work but not in the way you like. So that said if you have an app that hits the disk hard all day every day I would say this is an un virtualisable app as more often than not your other apps would like some time with the disk.
I agree that resource contention will still occur if you have, for example, 2 very disk intensive VMs on the same physical machine. If we intelligently allocate these VMs to different physical resources then they should work OK.
The notion I was getting at is whether certain applications can be virtualized at all given the overheads & limitations of virtualization. I'd be interested to hear about any "can't be virtualized" applications, assuming that they are the only VM running on the physical host so that there is no direct resource contention issues like the above case.