At first, I expected very little from this episode. I expected the appearance of Abraham Lincoln to be gimmicky, and combined with the whole "forced to fight to the death while the ship's crew watch on the viewscreen", which had been done before in a number of other stories, I really wasn't expecting much.
But to my pleasant surprise, "The Savage Curtain" turned out to be one of the much better episodes from the tail end of the series, and I am surprised that it isn't held in higher regard amongst most fans.
Unlike many late Original Series episodes, where the plot is pretty much laid out in the first five minutes, I liked how the story of this one gradually unfolded. Was this really Abraham Lincoln? What is he doing in deep space? The explanations, and the leading on to the forced battle on the planet below, are well paced, and I didn't feel that the story really dipped at any point.
As well as Lincoln, we also meet Surak, the "father of Vulcan civilization", adding some more background to Vulcan's history.
Out of the villains, Colonel Green and Klingon Kahless aren't bad (well they ARE bad, but you get what I mean!), but the other two barely get a look in.
Back on the ship (which, of course, is endangered, to add some more urgency to the situation), there are some nice moments between Mr. Scott and Dr. McCoy; I would have liked to have seen more of those through the series.
As I say, I personally found this to be one of the far better third season instalments, to the extent that it could just as easily been from the first or second season. It might not look much on paper, but the final episode is a good one.