Humanoid in Disguise

Isn't it weird that whenever one sees a movie about extraterrestrials they are quite close to our anatomy?
When one looks at the enormous diversity of life on Earth and takes in consideration the fact that other animals use their brains to plan ahead, it should strike one as odd that most film-makers don't stray away from the human body plan.
This reflection makes me think about the characteristics that a space faring species should have. Though these obligatory features do not preclude any type of intelligence on creatures that do not have some of them, those handicapped beings are irrelevant to the discussion. I’m talking here of species that develop a culture sufficiently active to produce the foundations of space exploration.
Remember one thing throughout this discussion: the words limbs, brain and body are used as name handles so one doesn’t get lost in this speculative text, that is, they could be anything.
It comes to my mind that any species that as a potential to be intelligent has to be heavy enough to support a big brain. Microbe-sized sentient beings are pure fantasy as there would be need of a great number of biological units to make something as complex as a brain. On the other hand giants do have the size on their benefit, but due to the large distances between limbs and brain their reaction time tends to be slow.
Also, the quick-wittedness a species has depends on their sizes correlation with metabolism. A small creature will tend to spend most of its time foraging for nourishment and fleeing from predators so time to relax and think is scarce. Besides due to its small body volume/ body area ratio it would spend most of its energy trying to maintain its internal medium stable. Hefty living beings get the other end of the stick: tough their need for food or homeostasis isn’t so energy-consuming, the metabolism slows as the body volume/body area ratio augments. If the metabolism was any faster they would cook themselves inside out.
The basic is that small creatures haven’t got the time and big creatures haven’t got the speed. So something between 20 to 800 kilograms is fair game.
After the size hurdle come the sensorial organs, important for the creatures’ perception of the environment. For one, it has to be sufficient but not overwhelming, using the readily available media of propagation. So no 1000-eyed monsters that drill the bedrock, but also no senseless brutes flying through the air: a creature that has between two and six of a particular sensory organ is not too farfetched.
Limbs abide by the same rules as sensory organs: dragons can be intelligent but giant centipedes can't. An intelligent creature can have two types of limbs, locomotory and manipulatory, but it’s not unusual for limbs to have both functions. Locomotory appendages is what the creature uses to get around. So they must be sufficiently efficient in their task and not much of a hindrance when the creature passes through unfamiliar environments. A creature that has from two to eight well-sized locomotory limbs can't be too uncoordinated. Manipulatory limbs, the parts of the body that define a creature as intelligent, capable of shaping its world. Important features are mobility dexterity and prehensible extremities, so manipulation of objects can be precise and elegant. They have to be in a number not to cumbersome for the brain to regulate but not to few for the creature to be helpless: so two to six is a fair bet.
Brains aren't the sole purpose of a head, so long as a brain is not too far from its sensory organs and locomotory limbs it can be anywhere. Brains on the butt are unreal but a brain on one's back isn't.
One thing for sure: it has to be a very mobile consumer, and an omnivore at that. There is no challenge in munching on grass and there's too much at stake when bringing down only prey, so a creature that can eat a bit of both is more suitable to evolve into a space faring species. This also requires of such a type of creature (the reason why carnivores can't have technology) that they have to be fairly unspecialised: that the route these creatures took on evolution didn't close too many doors for the development of intelligence.

Now after this rant can you tell me with a straight face that a "gray" is plausible?
The only reason, as far as I can tell, for the predominance of humanoid aliens in films (though there are few, but notable exceptions) is the fact that production costs for the creatures you and I have imagined while reading this are too prohibitive for any real attempt.

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