If I were to make an informal list of face features that one right rig...
(starting with the most important)
-mouth open/close
(this is all Muppets have and they do quite a bit with it)
-eyeballs pointable
-eyelids movable/blinkable
(to do something besides the straight ahead stare)
-eyebrows poseable (there's probably more expression potential in the eyebrows than there is in the mouth)
-mouth shapeable (this is what everyone thinks they need for dialog, but I think people obsess about it too much)
After that there is a long list of lesser elements like tongues, teeth, cheekbone scrunches, nose flares...