"Where No Man Has Gone Before." It's quintessential Trek. Kirk makes a decision, takes a risk, and has to face the consequences of that choice.
Whether good or bad, STAR TREK up till now has been about the captain and the decision he/she has to make, as Bryan Fuller has pointed out. And this episode exemplifies that, Kirk wrestles with the difficult decision on what to do with Mitchell, whose condition was caused by a decision Kirk made. It's great drama.
Also, I wish they carried over the Kirk-Spock dynamic in this episode to the series proper. I liked how they didn't see eye-to-eye on the issue. And I'm glad they brought back a bit of this tension in the new movies.