To me, they were all interesting enough to go from episode to episode. Then again, TOS is my favorite of the Trek series, and continuity is sometimes what kills a good story. I'd much rather have a weekly adventure out there, on its own, without having to worry if they're saying something that fits with a couple episodes before or if they identify a ship's class correctly.
It had charm, adventure, fun, charisma. It brought you in each week (or each rerun) and you were along for the ride. TOS was mainly about Kirk (and later Kirk, Spock and McCoy) anyway, with the others being a good supporting cast.