It is a shame Gene Roddenberry pretty much left TOS in season three. The first two seasons of TOS was some of the best that Trek has done. A weaker season three really hurt the series.
Well, it was sort of a chicken-and-the-egg situation. Star Trek was already at death's door by Season Three, which is probably why everybody, including Roddenberry, already had one foot out the door. They could see the handwriting on the wall (i.e. imminent cancellation) and were busy updating their resumes . . . .