Oh, let me counts the ways...
-Buffy is a woman, a heroine, who isn't defined by who she dates, she's her own person.
- Btvs shows real female friendship (very rare, even nowadays) between Buffy and Willow. Also shows male friendship pretty well (this isn't so rare, but still commendable)
- Story arcs that last an entire season changed the way pretty much all genre television shows were written.
- When the writers had really good characters (and actors) they knew it and let them stick around, and gave them their own great story arcs (Faith, Spike, Anya, Jonathan...etc.).
- a spin-off show that's at least as good as the original series (almost unheard of.)
- sharp dialogue, some of the cleverest, funniest television ever, ever written.
- I can't name another show where I love so many of the different characters.(at least a dozen of them, damn it.)
- hot chicks (hey, I can be shallow...)
- hot guys (and also open-minded.)
- and this is the important one, it took risks all the damn time, all through its run- 'Surprise' 'Becoming Parts 1&2', 'Hush, 'Restless', 'the Body', 'OMWF', Killing off Tara/The dark Willow arc, 'CWDP', 'Chosen', 'Not Fade Away' right up until its final season it took risks like no other show did (name one other television show that hasn't settled into some kind of rut after 6/7 seasons...), they didn't always pay off, they annoyed fans (hey, some of these things annoy me.) , but they were never, ever boring. And that's, ultimatly, a good thing. 🙂
I can't think of any more right now, but give me five minutes, I'll have another list, it's that kind of show.