It had balls. TOS wasn't afraid to try something different. They weren't afraid of putting characters in uncomfortable situations or even having them be wrong sometimes. Sure, there was a lot of camp but even though Kirk fought a guy in a godzilla suit by making a homemade cannon and firing diamonds out of it what happened at the end? Instead of the hero vanquishing the monster he stopped, told the aliens who had put them in this situation to fuck off, and spared the gorn who would have happily killed him just moments earlier. Yeah, we got Spock's brain but we also got The Corbomite Manuver. Also the characters, they've tried to recapture the dynamic several times but there is no matching the Kirk/Spock/Bones dynamic.
There are probably, twenty to thirty episodes to TOS that to this day I will not turn off, heck sometimes I turn them on as background noise when I'm doing other things on my computer. Episodes that aren't good TOS episodes or good Trek episodes or even good SciFi. They're just damn good flat out story telling.
This isn't to say that the other series haven't had their moments or their great stories but I don't think that even DS9 managed to completely meet the bar TOS set.
save perhaps Mayweather, who seemed to be a competent helmsman. He was just ignored because, you know... he wasn't white.