I enjoyed "The Changelling" overall, but I think that it suffered from the second and third season tendency the crack some silly joke(s) after a horrendous event had occurred.
Nomad wiped out 4 BILLION people from a planet, and yet Kirk and company are "joking" about Nomad being his "son". Even as much as I liked "The Trouble With Tribbles", what did they think the Klingons were going to do to those Tribbles? It would be like beaming over a bunch of cats on a ship operated by a Canine race. They're going to be slaughtered, plain and simple.
Compare this with the tone of the first season ("THe Enenmy Within" nonwithstanding). For example, in "Balance of Terror", the end scene showed the effect on Kirk regarding weight of the events that had occurred, but also showing him to being forced to rise above it and adopt his "Captain" persona. No jokes were made because there was nothing funny about the events that has occurred. Also, let's not forget the "....What of Lazarus?" somber tone that Kirk had in the ending of "The Alternative Factor".
Don't get me wrong, the second season had some great shows, but I think that many of some of them had inappropriate endings for the events that and preceeded them.
Had "Space Seed" been produced during the second season, there probably been a joke at the end regarding those new engineering "beat down clubs" that Kirk used to whack on Khan in order to defeat him, thus diminishing the drama of that episode.
Just my two cents.