Frank Gorshin (Bele) and William Campbell (Koloth) stand out to me. I liked Gorshin as the Riddler, but didn't care for him in any other roles I can think of - including this one. I liked Campbell very much in any other roles I can think of, except this one. I don't know if his acting, or the writer (or director) were responsible for the watered-down Klingon that apppeared onscreen, but it just didn't work - for me.
The other thing that was . . . odd, were the background characters that would die in one episode, and reappear later on, either as the same, or another background character - Eddie Paskey, take a bow.