There can never be any surprising, unexpected changes or lasting consequences for the characters we really care about.
But Trek often served purely as an anthology show. They visited a new civilization each week and THAT was where the novelty was, not so much through long pre-planned character-arcs. I mean, having a show that is almost all bottle-episodes isn't necessarily a bad thing, even though these days dramas have shifted to mostly serial storylines.
TOS really didn't have a lot of lasting consequences for any of the characters. These days you'd probably have call-backs for Edith Keeler, Miramanee, and things like that, but they had Kirk eventually get over his traumatic experiences.
There were only a few isolated episodes that touched on their back-story or were particularly revelatory. Amok Time, Obsession, Shore Leave (with Finnegan) things like that.