In case you forgot: The Enterprise is doing some follow-up on a planet Kirk remembers fondly as a peaceful, primitive planet. In the intervening years, Klingons have found it and made a deal with one of the tribes and generally made a mess of things. Kirk ultimately decides to arm the other tribe as well. Also, Spock got shot.
An episode that takes place outdoors, without any awful racist stereotypes! And Spock gets shot in the first act? Nice to see the baddies have some decent aim for once. The episode starts off strong and stays strong, with good pacing and a pretty rough moral dilemma: save the hill people's lives at the risk of a permanent, escalating arms race, or hope that the village people (hah) will grow tired of killing? The Prime Directive is the big linchpin of this episode and the episode makes a pretty good case for why Federation (Starfleet?) even has it to begin with.
The episode also grasps with what, exactly, the Prime Directive means: should they give the hill people more guns? no guns at all? Sometimes there's no right or good answer and you have to wonder if Starfleet is making the universe a crappier, more violent place. It sure seems like that's what Kirk and Bones were thinking by the time they got to beaming up.
The episode is almost perfect, but not quite. Nona blows: she exists to wear a sexy Muppet costume, mess things up, and then die (thereby triggering an inevitable war/arms race on what was a near-perfect planet). It also spends a lot of time with "will he make it?" plot point ("plot point") with Spock, which might have been great tension back in the day but doesn't really age well. Of course Spock is going to make it.
Also, the way that the episode basically insists that an arms race is the one and only deterrent to total annihilation is grossly unsatisfying. Let's not try to arrange a meeting with the village people (hah) or rout the Klingons out of there, let's just give everybody weapons.