^Well, it was the 1960s. Sometimes they could manage to do neat monster makeups, but usually their options for aliens were either humans with weird clothes and skin tones or animated light blobs.
There's also a striking amount of telekinesis in TOS, because it's very inexpensive to have one actor hold out his hand and another actor mime being affected by some invisible force.
I searched the transcripts of TOS, and the only references to life forms that are "not as we know it" are in "The Squire of Gothos," "The Devil in the Dark," and "Operation: Annihilate," plus a reference in "Return to Tomorrow" to "life there as we understand life." Now, while "Squire" and "Return" both do involve incorporeal intelligences of one sort or another, in both cases the lines are referring to the ability of the environment to support
life as we know or understand it, delivered before the incorporeal intelligences are encountered or revealed as such.