User blog comment:FTWinchester/Crosses, Holy Water, Bible, etc.../@comment-4199083-20140126052714

I just think the show is employing traditional vampire lore since the show in itself is a subversion of the horro genre, depicting various things like vampires, zombies, witches, Frankenstein's monster etc. so Buffy, based on the cliche of the dumb blonde victim, can literally "slay" those cliches.

On the subject of what gives those items power, I can't say. It's really up to the writers.