I am going from memories nearly old enough to drink.Tiny quibble on the one above - Ellison was asked to change the catalyst to McCoy and did so in his second draft. (I just re-read the book.)
The REAL reason that we remember this episode 55+ years later.Shatner's performance had a lot to do with it too.
Not anywhere close to controversial (it better not be!) I think Nimoy's performance here is almost underrated. I cannot think of any hyperbole that would be too much. Even though he is clearly a supporting character in this episode he is so good. He sees all of the outcomes clearly and they all make him miserable. Especially the ones that end with him staying in the 1930's. It's strangely one of the most natural performances that Nimoy gives as Spock.
It was, to be sure. But nobody ever saw this and said "It's so out of character for Harlan! He's usually so agreeable."I just reread the script and not the intro rant, but it seems to me Harlan got more pissed about Gene lying over the years than the actual script kerfuffle. THAT was totally justified.