Comment 1 for bug 1299784

Found the dconf setting which makes this available again (org.gnome.desktop.wm.keybindings.panel-run-dialog). For some reason it was set to ['disabled'] on my test system - no idea why. Setting it back to its default value makes Alt+F2 show the Run dialog again.

So this bug report is now merely about making this available by default, or making it configurable in GUI. Also, might be useful as reference for others having the same problem :-)