The most helpful aspect of KVM is the fact that the interface is so minimal. It includes just what you need to set up the VMs and manage them, and it's very simple to do so.
Documentation and problem-solving troubleshooting are the most valuable features. Performance (when fine-tuned and with "special" HW) is awesome, equal to or more than other enterprise closed-source solutions.
One problem I have is that it's not very scalable when it comes to resizing the VM disk dimensions. For example, if you have initially set a virtual drive to 10 GB and you want to upgrade it to 15 GB, it's not that easy.
