The tendency for all sci-fi alien species to be one facial feature away from humanity.
…
Gene Roddenberry gave more reasons for this in an interview once. Budget constraints aside, if you try to make aliens look completely alien, you’ll firstly make them look ridiculous (cf. Doctor Who), and secondly make it doubly hard for the actor playing the alien to do anything mildly resembling acting. This has actually been isolated to extremely specific requirements: if an audience can’t see an actor’s eyes or mouth, their ability to empathize with or emotionally invest in that character is significantly impaired. This is one reason why Mooks, especially SF mooks like the Cylons or the Imperial Stormtroopers, are so often uniformed in face-obscuring helmets.
—tvtropes.org, completely destroying your productivity one fascinated click at a time
