To explain the appeal and longevity of the original Star Trek and the franchise would take a thesis and hotly contested thereafter. It did not pioneer Sci-Fi on television, but its ideas, imagined by Gene Roddenberry, outlived the three short years network television allowed. Rather than hypnotizing viewers with special effects limited at the time, it focused on the stories and characters becoming a futuristic space opera with an occasional phaser and fisticuffs.
Rather than sticking to one, repeated formula, the series experimented with a variety of backdrops and morals. From exciting space battles to drama of revenge, to even comedy, Star Trek kept a core team together as they visited fascinating worlds across the galaxy.
The original special effects is dated. The acting at times is over dramatic. The techno-babble is eye rolling. But most of all it is fun! There is no television science fiction franchise comparable today after so many decades starting in 1966.