Here are some of my thoughts
First, we can agree that there are some holes in this plot. Yes, Spock's behavior reverts to his Vulcan ancestry whilst McCoy's does not. And yes, perhaps Mr. Atoz did not need to stay after everyone else had gone (although have you ever known a librarian that would close the library early if it was a slow day?) But how could everyone miss the first problem that I see – the Enterprise is coming to a planet that is about to be destroyed, to evacuate, presumably to their star ship, the entire planet's population (Thousands? Hundreds of thousands? Millions?) which will be destroyed by a supernova - in just over 3 hours? I mean, I know they can use the shuttle craft for the overflow crowd, but still. (Oh wait, if there are too many, Kirk will have to play Kodos the Executioner all over again?)
All that aside, this is a great story. They were able to blend the concept of time travel into a story that is essentially a tragedy – Spock finding that rare place where he can spend his life with a woman who feels much like himself (Zarabeth: "Do you know what it is like to be alone? Really alone?" Spock: "Yes. I know what it is like.")
Like all fine stories, this one has many fine ideas just below the surface. The two most interesting ideas that I saw both involved Spock and Zarabeth. First, of the 4 primary characters that went back in time(Kirk, McCoy, Spock, and Zarabeth), only Zarabeth and Spock were changed – Zarabeth was changed by the Atavichron, Spock by his "connection" to his Vulcan ancestry. And because they were changed, they could both start a new life, different from the one into which they were born, and find happiness, on that island of solitude, with each other.
The second idea is the one involving tragedy (fitting, since the episode's title comes from one of the greatest tragedies of them all). Zarabeth and Spock both shared tragedy. Zarabeth's was one of exile and solitude. She will never know love, physical affection, motherhood (the one thing that makes all women complete). And she would have had the chance for that, and more, with Spock, were he to remain. And she realizes this, even to the end (the single tear running down her face). And no less a tragedy is visited upon Spock. He is in love with Zarabeth, in a way that he could never be with any other woman, because the ancestral connection that is growing within him is allowing it. If you doubt this, just remember the last thing he wanted from Zarabeth – a moment alone with her to say good bye. ("How much time do we have?") He even tries to throw McCoy into the portal ahead of him. But he cannot – they came through together and must return the same way. He can't even have that small, private moment with her, just the two of them. Tragic. And once Spock gets back, he is back to being trapped in his world of logic ("She is dead and buried, long ago."), but with the memory of another type of life he could have led, one filled with warmth and love (ironically in the midst of a frozen wasteland).
One more thing: The final shot of the Enterprise flying away, leaving the nova and the destruction of the planet behind, was one of the best images of the entire series. I agree with those who believe that it was this episode should have been the series finale.
Finally, there is some great writing:
McCoy: "Spock, you pointy-eared Vulcan!" ...Spock (grabs McCoy by the throat): "I don't' like that. I don't think I ever did. Now I am sure."
Magistrate (pompously and officiously): "I'm here to see that you get a fair trial in front of the Inquisitional Tribunal." ... Kirk: "You must help me get back to the Library." ... Magistrate: "I can't." ... Kirk (a serious look comes upon his face): "Then I'll denounce YOU to the Inquisitor". ... Magistrate (pompous look is gone, replaced by one of horror): Please. I beg you. They will burn me."