Shortcut for "run application" in Unity
Install gmrun
sudo apt-get install gmrun
Use the Add button in gnome-keybinding-properties to create a new action with the command gmrun. You can then bound it to Alt+F2 or Super+R or any other combination.
This however is not as powerful as the Alt+F2 of the gnome-panel.
Other alternatives:
Install kupfer
sudo apt-get install kupfer
Install gnome-do
sudo apt-get install gnome-do
Install Synapse
sudo apt-get install synapse
This is a known bug: https://bugs.edge.launchpad.net/unity/+bug/580295
Try this: Five alternative apps for ALT+F2 functionality in Unity