Mary Elizabeth Williams argued in 20 Years on, "Angel" Has Aged Better Than "Buffy" at Salon that not only has Angel, the Buffy the Vampire Slayer spinoff, aged better, it is the better show. Those of you who know what I'm talking about will have some strong opinions about that, I'm sure.
Quite apart from all that, Williams says something in her article that made me think of the difference between YA and adult literature. Buffy, she says, "famously made real the universal truth that high school is hell" while Angel is a "workplace TV show."
Theme is supposed to be a big factor in identifying YA as YA. Thematically Buffy is about young people finding a place in society. Angel, on the other hand, is thematically about adults living in that place in society. It's not just that Buffy is set in a high school and Angel is set in an office that makes one show YA-like and the other adult. It's what Buffy and Angel do in those settings.
"Growing up is a finite process," Williams concludes. "Adulting goes on forever."