"The Conscience of the King" is one of my all time favourite Trek episodes. But you're absolutely right; having a balance is good.TOS could be solemn and did some damned good work with episodes like "The Conscience of the King" and "Obsession", but managed to balance it out with episodes like "Arena", "Errand of Mercy" and "The Man Trap".
I guess what I'm looking for is a balance that has been missing from Star Trek for a long time.
I find it funny how often people contrast TOS and TNG as if they would be somehow drastically different, while lumping all the 24th century shows together. TOS and TNG are my two clear favourite Star Trek shows (way above others) and I don't find them that different at all. Sure, TOS may be a bit more action adventurish than TNG, but to me it is more a difference of degree rather than kind. I find DS9 to be much clearer departure in tone and subject matter.