Every time I come across this sort of thing, I stumble. Not because I don't get what they're saying, but I wonder what boundaries they're intending to draw.
What makes an alien 'alien'? While 'does not look human' is an obvious thing, here's my cat Cinnamon. She doesn't look particularly human either. Does that make her 'alien'?
Cinnamon and I are not much alike. She likes racing madly up and down corridors, scratching things, and has a magnificent twitch reflex should a string move suspiciously anywhere in her field of view. She eats stuff that would make me vomit. She has no command of English, though she's a rather verbal cat, and won't hesitate to meow loudly until her humans try to guess what she wants. She isn't into computer games or reading.
And yet Cinnamon and I and thousands of other creatures are very alike. Earth itself is full of things not so very unlike Cinnamon, and yet all theoretically different from each other. All of them share a drive to reproduce (or, well, we wouldn't know about their species at all). All seek food of some sort, and an ideal environment. The more complex ones generally exhibit fear, aggression, curiosity, play and something that at least appears to humans as affection. I don't think anyone is arguing that 'aliens' could not exhibit these qualities.
But these are animals, and not at the intelligence level of the type of alien that SFF readers are talking about – a being that would be able to communicate effectively with humans if given access to language lessons and whatever mechanical aids might be necessary, while remaining 'truly alien'.
I think we can effectively divide aliens into 'physically similar to humans' and 'have a strong dissimilarity to humans'. Cat-ancestry people, for instance, might claw stuff and have a tendency to pounce, but they're still mammals, with live births, milk feeding, and the food-in/poop-out process. Then you have, say, the 'piggies' from Orson Scott Card's Speaker for the Dead (where reproduction is…different from the standard mammalian process) but otherwise these are creatures that exhibit a similar range of drives and emotions to humans. Thirdly the 'completely different' – living rocks, or energy beings that have little to no concept of many human physical experiences – but could still presumably produce fear, aggression, curiosity, play, affection. No-one is arguing that 'truly alien' aliens must not have emotion, or biological imperatives. If you don't go in for large physical differences, a lot of what people point to as 'alien' seems to be "non-Western culture" – and if you got into the alien's POV, they would read very much as humans of a different culture.
Different cultures are very interesting! Altered experiences caused by physical differences are also very interesting! But surely we're not arguing that beings with cultural and physical differences are intrinsically non-human.
Cognative differences takes us into more complex territory. Humans have cognative differences too, and I'm sure no-one who is talking about "alien aliens" means to say that non-neurotypical humans are 'alien'. Where does the line get drawn? We have humans with synesthesia. We've seen robots depicted with depression, or aliens who in theory have no concept of lying, or don't understand death or love or friendship. And, to be honest, most of this latter type of 'alien' reads as extrapolated concepts to me, constructed to create a plot. I am vastly, vastly more inclined to believe that if we somehow overcome the echoing hollows of space and stumble across a few intelligent alien species…we'll get a bunch of creatures that look an awful lot like Earth creatures – and probably act like them as well. Aliens because 'not from here', and with some physical differences, and big cultural differences, and not 'truly alien aliens' at all.
What do you all think? If we started planet-hopping, would you be all that surprised to find mammals? Cats? Bipeds with roguish grins, a taste for scotch, and a nice line in leather jackets? At what point do you throw up your hands and cry: "These aliens may as well be human!"?