Edit: Christopher and I were writing at the same time. I see he made a lot of good points about the story itself.
Both of you make excellent points, many that I wouldn't have considered while watching the episode. I'm particularly ashamed that I didn't notice the huge leap in the perceived danger between highly volatile to "end of everything" cataclysmic.
I'd heard about John David Barrymore having been originally cast as Lazarus, but not the subsequent on set problems between Robert Brown and the cast. That's interesting, and makes sense in particular with the scenes that Brown and Shatner have together.
One other question, and this is one of the few problems I come away with when watching this episode: Where are Sulu and Scotty? Shore leave? Seems odd not having Sulu at the helm or especially Scotty in Main Engineering.... considering that so much of the plot figures on that section of the Enterprise!