I'm really surprised at the relatively low rating of this episode since I found it to be one of the better ones toward the end of the third season. In my opinion, episodes like this one are the kinds of parables that Star Trek often does very well by masking contemporary problems and projecting them onto another culture so that people can see things more objectively.
The most intriguing issue was the society in which the difference between the rich and the poor has widened to a frightening level, and in which the inequality has become chronic. I especially like it when the high commissioner guy makes speeches about how the Trogolytes were free to achieve and live in Cloud City, but they were just too brutal, stupid and unmotivated to do so. In other words, "They get what they deserve." They could do it if they want, but they just don't care.
I hear these kinds of arguments being made about other groups of people all the time, something like: "Those people living in the ghetto brought it on themselves, they chose do not improve their skills, they're just lazy and stupid. I'm not preventing them from achieving, and they want to live this way. They get what they deserve."
I often hear these kinds of arguments from people who were born with all the privileges of an upper-middle class upbringing, who were taught from the time they were a kid how to achieve in the world by their parents. The kids who got everything paid for to go to the best colleges and were groomed to join the "old boys network" from the time they were 2 years old. Surprise! They succeeded and are living a comfortable life. Strangely, they want to take all the credit for making all the right choices in their own lives and dismissively pass off anyone else's problems as "those other people brought it on themselves."
Meanwhile, I question if these champions of "they got what they deserve" would be doing nearly as well if they were born in lower circumstances. Some people certainly transcend their circumstances, but the high commissioner character hits this kind of arrogant, smug self-satisfaction right on the head.
The "gas in the mines" issue is a little less satisfying, but creates an even more drastic circumstance hindering the development of people in that stratum of society. Certainly the environment that many people have to put up with is an obstacle to their success.
Star Trek is always good at explaining the need and will of humans to be free and achieve (in about every other episode there is some speech about it) but it also gives compassion and understanding and tries to point out obstacles and social inequalities. I thought that this was a pretty decent episode about entrenched social inequality.
This kind of social inequality still seems especially applicable today since the gap between rich and poor is wider than it was even when this episode was made.