After commenting in a couple of other threads about various TOS episodes that speculate about the ST universe, it struck me that TOS had a certain storytelling technique that the sequel series didn't practice too much: Not telling us everything.
When they were first creating Star Trek, Gene Roddenberry & co. decided against giving us explicit explanations about how certain equipment worked. After all, the cops on Dragnet didn't stop the story to give us an explanation of how their firearms worked, so why should Kirk & co. do so with their phasers or warp drive? The viewers didn't really need to know how the phasers worked. It was enough that the characters did.
This led to TOS sometime dropping tantalizing references to things that were common knowledge to the characters, but unknown to the viewers at home. What was the Tantalus Field Device and how exactly did it operate? What were the Vulcanian Expedition or the Axanar Peace Mission? How old was the UFP implied to be? What happened in the Eugenics Wars when the supermen simultaneously seized power in 40 nations?
It strikes me that dropping references like that is a great way to engage the imaginations of fandom. Sir Arthur Conan Doyle did a similar thing in his Sherlock Holmes stories, dropping intriguing references to cases that the readers never were never witness to. People have been speculating about the Giant Rat of Sumatra for over 100 years now. I think you could make a good case for Star Trek fandom being birthed in those almost-throwaway references that don't explain everything. The subsequent shows, almost unavoidably, became a bit more about filling in the gaps that already existed instead of creating new ones. I think this might be part of what keeps me coming back to TOS when I've lost interest in some of the other shows. (What's more intriguing, being outright told that the Federation was founded in 2161, or to hear how they only had subspace radio during the Earth/Romulan War?)*
*Please note that I DON'T mean this thread as a pointless exercise in bashing the subsequent shows. Heck, you can find examples of this sort of worldbuilding in the more recent Treks. A throwaway reference to the "Rules of Acquisition" in a DS9 ep became the cornerstone of developing the Ferengi culture. It's something that I hope future Treks remember to do. It's nice to have an occasional reminder that we don't know everything there is to know about the Star Trek Universe!