The use of Unity as the default interface for later versions of Ubuntu is a welcome addition for some as it makes better use of limited screen sizes. But when Unity 2D was discontinued in Ubuntu 12.10 it caused problems for people, like myself, who keep a virtual Ubuntu installation at hand for those tasks that require a Linux system. Running Ubuntu 12.10 or later on VirtualBox will often result in terrible performance, making it virtually useless.
One solution is to stay with Ubuntu 12.04. But another solution is to fallback to the old interface. Luckily, this is very easy to do by installing the package gnome-session-fallback.
sudo apt-get install gnome-session-fallback
After having installed the package just log out. When logging in select the Gnome Classic interface from the login page. Subsequent logins will use the same interface as the previous session. It is also possible to change the default interface.
sudo /usr/lib/lightdm/lightdm-set-defaults -s gnome-classic