As the joke goes, there are about 30 good TOS episodes out of 80. Most of those were in the first year of the series.
There's also the "low-hanging fruit" aspect: do the Planet Run By Computers, do First Contact, do the Mad Scientist, do the Time Travel episode, do the Space Combat episode, etc...
TOS became repetitious in year two, and relied more often on the popularity of the characters themselves to carry stories.
I once wrote an episode guide to a fictitious TV series. I managed to put together an entire twenty-plus-episode first season just by running through all the stock plots: the Ten Little Indians episode, the Most Dangerous Game episode, the Evil Twin episode, the Amnesia episode, the one where the heroes get stuck with a baby, etc.
It was scarily easy!
