I think aspects
of TOS haven't aged well. Beyond the obvious question of visual effects, and the slightly less obvious question of what kinds of pacing and episode lengths are now standard, I think it's safe to say that TOS's depiction of gender roles is very outdated. The role of women in Starfleet and in society, and the constant "male gaze" and male-dominated POV is very much a function of the 1960s and their attitudes towards gender roles.
TOS's depiction of race relations isn't as bad, but still isn't quite up to modern standards. The society TOS depicts is clearly one in which racism and oppression are supposed to have been vanquished and equality established -- and we do see authority figures and very accomplished people who are persons of color -- but the narrative itself is still crafted around white people, and the society it depicts is still one where whites seem to outnumber non-whites (even if you're alien). So the narrative is unintentionally ethnocentric -- not an unforgivable sin, mind you; its heart is in the right place. But it's also still a creature of its time.