As somebody who did
come to TOS late (I was a TNG child yo), I came to appreciate it by putting it in context of it's era. Place it alongside the sixties Batman
series, Get Smart
, things like that. The 'corny' aspects are perfectly palatable in that context. TOS is a carefree, fun series that doesn't take itself too seriously most of the time. It arguably doesn't have the layers of some later instalments of the franchise, but what it lacks in those departments it more than makes up for with it's sheer, unadulterated sense of 'joie de vivre'.