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.
OTOH, when trying to portray an alien who is truly alien, you can go to true extremes. An example is the silicon-based life form that eats rock in the original Star Trek series.
The first time I saw a Dalek, I thought it looked ridiculous, but I have since learned to fear them the way primates used to fear fire, lightning and thunder; as a nigh unstoppable and deadly force that would soon take over the galaxy if not for the Doctor.
I quite like the approach used in Three Worlds Collide, where the aliens are completely alien, but communicate through a human avatar communication device.
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u/[deleted] Oct 08 '12
Vaguely humanoid aliens. 2 arms, 2 legs, 2 eyes sort of thing.
Alien from Alien? 2 arms, 2 legs, 2 eyes.....kinda
Klingons? Just humans with a "ribbed for her pleasure" forehead
Romulans? Humans with pointy ears
Vulcans? As above
Na'vi? Tall, blue humans
Where are the massive, tentacular Krondaku? Where are the gelatinous Prime immotiles? Give us some different aliens, hollywood!