The way television is written and produced was revolutionised by Buffy.
The way a show is structured, interactions between characters, etc.
Most popular teevee wouldn't be what it is without Buffy.
Buffy isn't cult - the following may appear cultist, but it's huge. It's just been a while.

The cultural significance of Twilight is the same as any other mainstream juggernaut - money. If it makes an actual impact on people (I cannot tell you how many people I know who view Buffy as a life changing text) then there's something to talk about.