While this episode may not depict any monumental development or integration of character, I am happy to say it is not a regression. The forms of our crew, so well defined in the prior episodes, carry over strongly into this discovery-based adventure.
The exploratory nature of this episode is classic Trek. The crew is pitted against a life form that couldn't be more molecularity different from traditional life. This silicon based "Horta" creature is attacking workers in a mining facility that supports scores of planets with necessary minerals. The conflict is high as the Horta steals an integral piece of the facilities life support. A time-based enhancer of tension though it may be, it allows for an escalation of conflict that necessitates the intervention of our heroes. An instigating event that is the first clue on the path of discovery.
Here Kirk continues his role as moderator. Whereas the miners would be perfectly content destroying the creature they don't understand, Kirk is able to negotiate a treaty with the Horta. (With the help of a mind-melding Mr. Spock, of course.) It is this continuation of the theme of unity and reconciliation that makes the episode uniquely Trek, elevating it beyond the simple discovery of the unknown. It is the furthering of relations, communication, and understanding that take center stage. Through Kirk's uniquely arrived at conclusion, the facility is now able to produce record minerals with the cooperation of the Horta. (Which, as it turns out, was a mother protecting her children.)
Messages of tolerance of this type will not end with this episode, and become a staple of what defines Trek in the minds of so many. Thanks to such vividly drawn characters, we are able to be continually reminded of what they represent in the face of our own conflicts that often mirror the outrageous space-age adventure depicted in Star Trek.