Bones hardly goes an episode without pointing out how Spock is not human, and that makes him inherently inferior. They are clearly friends, but Bones is casually racist. The striking part is that I do not think a single other member of the crew ever does anything similar with regard to any species.
This alone is a strange and striking aspect of TOS, but in TNG it becomes a pattern. In TNG Spock is replaced by Data, and again the medical doctor of the enterprise is the only character who does not believe that a mechanical being can be alive, conscious, or matter. Every series after Gene Roddenberry's death features doctors with, arguably, bigger roles and who continually show themselves to be the most empathetic and open minded of their crews.