I was a little puzzled after upgrading to Ubuntu 11.10 to notice that the Alt + F2 keyboard shortcut was not working. It turns out, it wasn’t removed, it was just disabled. This is how to re-enable it.
Open the System Settings from the top right of the desktop.
Under Hardware, go to Keyboard. (You can also access these keyboard settings by searching for “keyboard” in Unity and Gnome Shell’s respective “dashes”, which I did in the video at the bottom of this post.)
Next go to the shortcuts tab. Under System, select ‘Show the run command prompt’.
Click the word disabled to edit. Once clicked, ‘New Shortcut’ will appear for you to select a key combination.
Press Alt + F2 to set that as the keyboard shortcut to launch the run command prompt.
You should be able to use Alt + F2 again.