Wow. Without going into barf-inducing technobabble, we get discussions of the fabric of space leading to other dimensions, a cool new alien race, interphasing between the dimensions, the area harming organic life, and Kirk being caught.
I'll pretend the one gaffe (the Defiant ship is said to be 'dissolving' yet people can still walk on the floor) is due to parts of the ship going into interphase but the floor is still intact...
The breakdown of the crew as time goes by is definitely stunning. Having Uhura, Scotty, and others see Kirk and treated as if they were breaking down as well is terrific. The idea of being caught alone in a universe is quite novel as presented in this story.
And then there are the Tholians. "The screensaver race"; I've always wanted a screensaver of a Tholian for some reason. I like their appearance, language, their idea of making a web to fling incapacitated ships out of their domain (instead of simply destroying them - how much moral can a species get?)
Definitely a classic, and needless to say a triumph for the much maligned season 3.