Most depictions of aliens are essentially just humans.
Special effects limitations are partly to blame, but the phenomenon extends beyond live-action TV, so it's obviously deeper than that.
The anthropomorphization of aliens is, aside from being fallacious, terribly boring.
I generally dislike it.
So you want to go beyond the anthropomorphized ideals and design something truly alien.
The obvious approach is to take inspiration from the variety of life on earth.
Animals, bugs, plants, fungi, bacteria, etc etc.
Anthro constraints now set aside, we're presented with endless new parameters for tinkering-
All sorts of alternative body plans, sensory systems, reproductive strategies, and so on.
Here we encounter problem #2.
When one discovers all these interesting variations, there is a tendency to treat the traits as free parameters, to mix-and-match as we please.
Why not make our aliens reproduce asexually, and walk with 13 legs?
While we're at it, let's make them silicon-based, and give them a calcium shell, from which extends 3 clusters of 27 tentacle arms.
Just for funsies.
But of course, evolution does select traits just for funsies.
There is an underlying logic of selection pressure and environmental adaptation and whatnot.
We are aware, of course, that things are as they are because of reasons.
So, we go back to the drawing board, this time with evolutionary logic in mind.
How to determine, for example, the number of legs our species should have?
We think about our observed patterns of life on earth, and find a remarkable tendency for creatures in a certain size range to have exactly 4 legs.
This is easy to rationalize:
Bilateral symmetry demands the number be even, it's hard to balance with just 2, and adding any more than 4 would require more energy and advanced coordination without any clear payoff.
Thus, 4 legs are optimal, and we give our alien critters 4 legs.
You may have caught on to the problem.
Our logic can only be determined by post-hoc justification of patterns in Earth life, and thus the patterns of Earth life are all it can reproduce.
Any features which don't resemble Earth life will be bound to violate these patterns.
We've constructed a false sense of inevitability, and reasoned our way back to a sort of anthropomorphism.
This was on my mind throughout the run of Alien Biospheres.
Biblaridion's planet is populated by 8-legged critters, and covered in red plant-like organisms.
Some part of me can't help but feel that these stark differences from Earth life indicate that he's cheating somehow, that he's ignoring important evolutionary factors for the sake of artificially-crafted novelty.
Why haven't all those legs evolved away?
The vast majority of plantlife on Earth is green for a reason, you know.
Any distinction from Earth life begs the question, "Why are the selective pressures that normally result in X not applicable here?"
Maybe he has justifications I've forgotten about, but regardless-
I know, of course, that the final outcome of applying this criticism is an exact recreation of Earth life.
Something is amiss.
It's a special case of the troubles with representation in general, I think, the issue with conceptualizing all difference as difference with respect to some transcendent ideal rather than difference-in-itself?
Something like that.
Returning to the example of number-of-legs:
When reading carefully, there is an obvious back-door to the line of reasoning I presented.
I analyzed only creatures in a certain size range.
Earth is actually filled with many-legged creatures: bugs.
For some reason, smaller critters tend to have many more legs.
There is a yet-deeper underlying logic one can pick apart, but we must tread carefully.
If we simply adapt our logic to make leg-variability a function of size (or whatever pattern we happen to find), we are still reproducing the problems I discussed.
We will end up with 4-legged large aliens and many-legged small aliens... exactly like on Earth.
We need to adapt out framework in a deeper way.
The broader takeaway, I think, is that apparent evolutionary inevitabilities are actually dependent on hidden variables which we might be taking for granted.
Pay careful attention to the axioms of the arguments.
Remember that selective pressures don't exist in some transcendent, context-independent way.
Organisms adapt to their environment.
Thus, if your alien environment is not Earth-like, that should (hopefully) be reflected in the evolutionary outcomes, even when applying the same Earth-derived logic.
Of course, since Earth is our only point of reference, any divergence into properly alien territory will be highly speculative.
That is a fundamental issue.
It means that your radically alien evolution is likely to be unfalsifiable-
in science, that's bad, but in worldbuilding, it's a genuine source of creative leeway.