I'm not reading any actual objective reasons why you think this is an effective episode, so maybe it's more of a personal favorite. At face value, it's more cobbled together scenes than an actual drama, and ultimately feels poorly executed. I dont know how Frontiere scored it unless he took a "library cue" approach.
With me it's always the writing, first and foremost. Stefano let it all hang out here, creating very definable characters, putting them in the boiling pan and exploding their cracks in very believable ways for each character. The twist that it's a test is just a cruel joke at that point, and it's foreshadowed early in the story with the shot of the generals watching behind the glass with the Ebonite, but a good character study story like this works for me even without that twist.
There is no moment of corniness in the dialogue; it's all totally distinct to each character, and the acting is totally up to the task of giving it the gravitas necessary for the 'nightmare' situation. Man, when I was a kid, I'll never forget actor Bill Gunn's moment when he talks about bargaining to get his sight back in return for seeing Krug's corpse and then breaking down saying "He didn't have a heart. There was just a hole in his chest!" That gave me some real chills and nightmares. Because when you have great writing and acting, you don't need to show everything. Gunn's reaction is more powerful and horrifying than any graphic scene of what he actually saw... A lesson lost on most media these days when CGI can just do anything and the creative ways writer's used to make you use your own imagination to complete the scene through dialogue or narrative alone is so easily and lazily abandoned.
I'm by no means 'objective,' but these are some subjective reasons this episode always grabs and impresses me. And there's so much more to come (this first season).