A legend is born under the watchful (but not unerring) eyes of Gene Roddenberry. Eventually, exhaustion would lead him to give the reigns over to others (some more successfully than others). But it was in this season that the tone for the franchise was set.
Some of the rougher patches came as the pressure to meet air dates mounted. But the quality remained very high for the most part, thanks to writers like Gene Coon, Steven Carabatsos and Dorothy Fontana. (A guy named Ellison wrote something, too.)