(Art: Cheesy_Cryptid. Purely for aesthetic purposes, doesn't perfectly match my concept.)
I imagine Cybertronians being rather alien, however, I don't particularly enjoy the Bayverse route. So, I am writing about my idea here!
(From this point forward, I will be writing specifically about my idea, which is why there are absolutes.)
For a beginning example, their speech, even translated to English, doesn't sound entirely human. It is incredibly strange and disjointed because it is from an internal translator and is NOTHING like how they naturally communicate.
Most of them do not have cockpits, and those that do are entirely metal and a form of living glass. They do not have leather and such like the inside of an actual car.
Their alt-modes do not match the size of respective Earth vehicles. If Optimus, for example, scans a truck on Earth, it will be a massive version of that truck, made of different materials.
Their biology is also entirely inhuman, and they only look humanoid on a surface level. Their mouths, for example, are incredibly strange and superheated. They only look human from the outside.
Energon varies in consistency, and it re-crystalizes over time when exposed. It grows, almost like a crystal fungus, and constantly vibrates.
They do not solely transform into their alt-modes. Their biology and structures shift constantly, to a limited extent, of course.
Cybertronians additionally have different proportions to humans, with rather short torsos, and long limbs.
Finally, for now, they possess a form of "landing gear" and pseudo-fingers on the bottoms and sides of their pedes, for grip and balance.