All Our Yesterdays is my favorite episode of the orginal
Star Trek; for so many reasons, a lot of them already mentioned in this thread. I know, there are some flaws in the logic of this episode. The reverting of Spock's behaviour is quite dubious, for one thing. The 'Kirk trapped in the Middle Ages' storyline is also something I could do without.
But there's one thing that this episode has going for it and that's the relationship between Spock and McCoy. I love their scenes together. I'm also fascinated by the backstory of Zarabeth, who was sent back in time by a tyrant and is forced to spend the rest of her live in cold isolation. (It also helps that she is so skillfully portrayed by the beautiful Mariette Hartley.) In the end this episode tells us so much about Spock, one of the most intriguing characters in all of
Trek. Only after he is sent back in time several millennia in the past and has lost the confines of his Vulcan logic he can find love and companionship. Talk about isolation, eh ...
Last but not least I'm also quite fond of the dreamlike, melancholic atmosphere that is created in this episode by the cinematographic imagery of the snowy past of Sarpeidon. In my opinion,
All Our Yesterdays is one of the best looking episodes of the whole series.
As for how well this episode works as a series finale ... Live
ssosmcin, I always felt that the last shot of the
Enterprise escaping Beta Niobe going supernova had the quality of a good farewell for the series as a whole.