I think the main problem with Buffy is that its fans make claims for its importance that far outweigh the actual worth of the show.
I mean, I liked it. Mainly for the structure and pacing of the season arcs than for anything else, but it was a genre show with spark and personality, and it was very well constructed - especially when compared to the meandering awfulness of the X-Files decline. But Whedon does have a problem with dialogue - he’s very good at writing the faux-witty one-liners, but he sometimes struggles to give his characters distinct voices. The thing that brought this home to me was his work on Astonishing X-Men, where you could take much of the dialogue out of context and not be able to tell if it was from Buffy or X-Men, or even who might have said it.
When people say patently absurd things like “seriously, this is one of the best shows that came on the air ever” then you’re writing a cheque that the show can’t cash.
It’s a good show. An occasionally inspired show. But it’s far from one of the best shows ever, and it’s that sort of hyperbole that helps foster a binary love/hate reaction in new viewers.