Pretty much all films and tv shows age very rapidly. Some of the things that made the show very progressive at the time, seem out of step with how things are looked at today. What keeps the show fresh in my opinion is how much they played with the horror genre, the humor, the character archs. I think the show holds up fairly well, even though more things stand out now as odd choices than they did at the time. Xander, for example, is a highly toxic individual and I enjoy his presence less every time I rewatch the show. Seeing the shift from the asshole nerd as sympathetically portrayed when Joss was the showrunner to the nerd being portrayed as the villain when Marti Noxon became showsrunner is… interesting to see in retrospect though.