Personally I like the Hellmouth.
Perfect plot device for 'Monster of the week' in a small town setting. It meant they could build up the town, its inhabitants, and all its locations to have an intimate feel to the setting, but still explain why weird crap always happened in a small Sunnydale town (and had apperently always happened here since its founding)
I also like the darker aspects of the Slayer that were always hinted at throughout the series- the hunting instincts, the urge to kill, the love of combat/violence, the...primal urges (Hungry and horny basically- because sex and death are closely linked in the human psyche anyway- surviving something bad makes you want to feel alive, ask any soldier.) and all the other things that seperated the slayer from the humanity they fought to protect.
It gave the protagonist a nice shade of grey- more fully explored with Faith than with Buffy- but Buffy shows all those same instincts, just not to the same degree. 🙂