This was an episode that I kind of got burned out on as a kid, for some reason it came up in the rotation a lot on my local station. I always liked it, though; the Tholians and their motives seemed suitably alien and the Spock McCoy scenes worked well. I agree that the music was very well done, even if none of it was original. The "ghost story" scenes with Kirk appearing were maybe over-played a little, but they work OK for me. And hey, space suits! I thought that was so cool when I first saw it, though the helmet design seems a little odd.
Interesting, how the producers agreed upon naming Enterprise and her sisterships after World War II aircraft carriers (Constellation, Lexington, Exeter, etc.) and yet they forgot their naming scheme and named this derelict Defiant.
Well, Exeter wan't an aircraft carrier name, so that was the first variation in season two, followed by Hood, Potemkin and Excalibur in "The Ultimate Computer."
As a warship name, Exeter is best know for one of the British cruisers that ran down the German raider
Graf Spee early in WW2. Though she was not the flagship of the task force, she did take the most damage and inflicted some crucial blows on her adversary. Just speculating, but the name's familiarity in the US may have been helped by the 1956 movie
The Pursuit of the Graf Spee (original UK name
Battle of the River Plate). In the movie,
Exeter's damage is depicted memorably, with her captain and crew continuing to carry out their duties even after the bridge has been pretty much burned out. I don't know if it was shown on US TV in the '60s, but I'd guess it was. I do remember it on TV in the '70s.
To speculate more: Defiant was not a warship name with any history, but the choice may have been influenced by the 1962 film
Damn the Defiant starring Alec Guinness.