Star Trek: Deep Space 9 did many things to disappoint me. One of them was blowing the chance to fill out the Star Trek universe.
One example of this came in an episode where Captain Sisko's son, Jake, decided to do something nice for his father. Someone came onto the station with a rare baseball card. Jake wanted to get the card to give to his father. Jake's problem was that the Federation doesn't use money. In order to obtain the card, he had to get the card's owner something. That involved going to someone else to get from them what the owner wanted. Over the course of the episode Jake became involved in a series of deals, just to get the card for his father.
Since this episode came late in a season, when tensions were ratcheting up between the heroes and the Dominion, it was criticized at the time as being filler. However, it could have been an important story on its own. The story would have had one of two messages at the end:
One - Jake learns that all those deals are why the Federation has chosen to exist without a monetary system. At some point the Federation came to view money as leading its people to confuse needs and wants. Needs are now taken care of. Wants are harder to obtain, and either require doing what Jake did, helping others, or by being anti-social (stealing, killing, and such). This is why the Federation has no money, and explains more about it.
Two - what Jake went through causes him (and maybe Captain Sisko) to ponder the Federation way. Sure, needs are taken care of, but what about wants? Could Jake have gotten so frustrated that he gave up on his attempt to make his father happy? Could that frustration have instead turned to crime? Is the Federation way truly the best way? Does the system have to be changed, or is this a flaw that people have just chosen to live with, given the alternatives?
Either one of those two messages would have filled in a major detail about the Trek universe. Either one gives the audience a reason to think about how that universe it set up. Is the future portrayed in Star Trek one to strive for, or is there a flaw we need to be aware of as we create our own future?
There are times in fiction where the details can make a difference in a work. I try to keep this in mind when I create my own universes. I'm not always good at filling in the details, but I make the effort whenever I can. I don't want to make the mistakes that I find in other creator's works.