Unity is built using Compiz and requires 3D acceleration; a lot of people have issues running it in a VM. There's a Unity2D project built using Qt, but I don't know if it was mature enough at the time of release to make it in as a fallback.
[1]: http://www.theopensourcerer.com/2011/04/21/ubuntu-natty-in-v...
I only had to install the guest extensions to get 3D acceleration and Unity to work.