It's structure is also pretty good. Though most episodes are just an adventure of the week format, things are not forgotten between episodes. They are constantly cross referenced and characters develop across them, even in the most formulaic and seemingly throwaway episodes.

It's still far from the modern serialized standard (thank god I much prefer episodic shows) but it lacks one of the biggest pain points of shows of it's era.

EDIT: Except literally in the second episode, which I remember being the worst episode and so far (two seasons in) it's remained true.