Faces

Many aliens in sf have heads with recognisable faces, minimally two eyes above and a mouth, for drinking, eating and speaking, below. A nose and visible ears are optional but usually present. Ears may be pointed. Star Trek has Spock of Vulcan and Poul Anderson's History of Technic Civilisation has Aycharaych of Chereion, although Anderson does not commit the absurdity of suggesting that a Chereionite (descended from flightless birds) could interbreed with a Terrestrial. 

A fictional alien is generated, e.g., by putting a cat-like head onto a humanoid body. Thus, terrestrial features are projected onto extraterrestrial organisms as, in the past, onto supernatural beings. Closely linked to recognisable faces is easy communication. Like us, the aliens not only have faces but also use them for communication. Intelligible sounds emanating from mouths are supplemented by tones of voice and facial expressions as well as by familiar body language. We can learn each other's spoken languages. We may be friends, enemies or trading partners but in any case we understand each other. It is harder to imagine easy communication and friendship with faceless beings.

Probably all that can be said about alien intelligences is that, if they exist, they must have:

organs for perception and communication;
limbs for locomotion and manipulation;
orifices for ingestion and excretion. 

Anything lacking these functions is inanimate. Static, plant-like organisms do not interact with their environment enough to need to think about it so no motion or manipulation probably means no intelligence. Organs, limbs and orifices need not be immediately recognisable by us. They need not be organised in a way that we would recognise as bipedal, quadrupled, winged etc. The brain might be protected within the body or brain functions might be dispersed throughout the body. Organs etc should be identifiable by their functions if we have enough time to interact with their owners but their initial response to us might be fear, hostility, aggression, indifference etc.

 A bodily surface need not resemble skin, fur, feathers, rhinoceros hide etc. It will not have evolved in an exactly Earth-like environment. Too many factors could differ:

the mass of the planet, hence the strength of its gravity;
distance from its sun;
nature of that sun;
whether the "sun" is one of the fifty per cent of stars that have one or more stellar companions which might prevent the formation of any planets or at least of planets with stable orbits;
rate of rotation;
axial tilt;
the precise mixture of gasses in the atmosphere;
radiation levels;
the amount of free liquid on the surface;
whether chemistry on the planet is right- or left-handed;
whether there is a plant like grass that grows across the surface and that can be cropped down to ground level without being killed;
whether there is a large satellite whose gravity can thin out the atmosphere of the primary and can also cause tides, facilitating the evolution of amphibians and thus the transition of life from sea to land;
whether a quadruped climbed into equivalents of trees, stayed there long enough to develop opposable thumbs and came back down to the surface with forelimbs freed for manipulation;
whether such a biped was solitary and taciturn or social, thus potentially linguistic;
whether manipulation and communication could have developed otherwise;
whether predators, an ice age, a solar flare or a comet wiped out a promising species before it developed intelligence;
whether such a species has developed civilisation, then technology, and managed not to destroy itself long enough to contact intelligences in other solar systems.

The Burgess Shale implies that a small difference in environmental conditions could cause large differences in organic forms.

A long series of contingent events and processes was necessary to generate technological civilisation on Earth. Is it probable that such events and processes have been duplicated or paralleled elsewhere in the observable or even just the reachable part of the universe? There is as yet no positive evidence despite regular observation of heavenly bodies and, specifically, a Search for Extraterrestrial Intelligence. The expanding universe needed to reach a certain age, therefore size, before stellar fusion had synthesised, from the primordial hydrogen and helium, the heavier elements necessary for life. Thus, despite cosmic size, it is possible that we are the first intelligences. The discovery of several systems with super-Jovian planets near their primaries contradicts an earlier idea that the Solar System was a model for the formation of planetary systems, with at least one terrestroid planet in the temperate zone near the Sun and gas giants further out. A single datum from another planetary system is worth more than endless speculation.

Other optimistic sf premises are: 

easy faster than light interstellar travel;
humanly colonisable extra-solar planets (albeit sometimes requiring special equipment, precautions or dietary supplements).

Although I value Anderson's History, I doubt that our descendants will travel quickly or easily to other stellar systems there to meet equivalents of Merseians, Ythrians, Scothani, Donnarians, Cynthians, Wodenites, Chereionites, Diomedeans, Ivanhoans, Ikranakans, Starkadians (two species), Talwinians (two species), Didonians (three species symbiotically forming one intelligence), Martians (in this case, extra-solar aliens colonising a solar planet) etc. The non-humanoid Ymirites and Baburites might be less unlikely. Although many sf aliens may be as impossible as the gods of fantasy, we still enjoy such fiction, particularly when it is brought to life by Anderson's prose. The old quarter of a city on a colonised planet has:

  "...a brawling, polyglot, multiracial population, much of it transient, drifting in and out of the tides of space." (1)

Buildings are low because of Imhotepan gravity. The multifarious wares of the market place include a screen displaying:

"...an exquisite dance recorded beneath the Seas of Yang and Yin, where the vaz-Siravo [Starkadian refugees] had been settled." (2)

"Folk were mainly human, but it was unlikely that many had seen Mother Terra. The planets where they were born and bred had marked them. Residents of Imhotep were necessarily muscular and never fat." (3)

The view point character hears (Space) Navy men complaining about "Merseian bastards", then rushes to greet the first Wodenite, a giant centauroid, that she has ever seen. Such multi-species scenes are one glory of old sf.

 

(1) Anderson, Poul, The Game of Empire, New York, 1985, p. 1.
(2)  ibid, p. 3.
(3) ibid, pp. 3-4.