Yar.
Seriously, I re-watched it a couple of weeks ago, and it is some seriously awesome stuff. It's the balance of the morality. You're never quite sure which side you should be backing. There's also the bold reality of it all. Buffy, for example, doesn't quite seem to exist in a world where paedophiles or stock market scams could play a major part in the plot.
Oh yeah, and consistent internal logic, no plot holes, brilliant design, fantastic casting and acting, good music, great cinematography, subtle use of effects etc. etc.
The first two points alone elevate it above 95% of genre series.