What sets the original series apart for all time is the soundtrack, which at its best did as much to create atmosphere and help the story as any other element. I will always love the music in the engineering search scene ... this many years on now, the music also generates a lot of nostalgia for me -- I'm sure you too went around as a little kid humming Star Trek music like this.
Awesome score for this one, by Sol Kaplan. He's the guy who in season 2 did the score for Doomsday Machine, an even more awesome score. Those were the only two episodes he did original music for. But piece of his scores were "tracked" into other episodes, so you hear his music thruout seasons 1 & 2.